If unconcerned with enhancement bonuses and apenas needing the weapon special abilities, the 1st-level artificer infusions personal weapon augmentation [trans] (Configuração de campanha Eberron 117) et ai. e personal natural weapon augmentation [trans] (Raças de Eberron 188) et ai. grant weapon special abilities to one of the creature's weapons or natural weapons.¹ A wand of personal weapon augmentation (1st-level spell at caster level 1) (15 gp/charge) is inexpensive enough that even if adding the magic weapon special ability ousado (microfone 30) to one's unarmed strike every encounter, that will ainda be cheaper than having such as permanent magic weapon special ability using the methods below. The magic weapon special ability furioso (OA 125) is in a similar but worse position, as such an effect can only be granted by a wand of weapon augmentation [trans] (ECS 117) (4th-level spell at caster level 7) (420 gp/charge), likely making long-term use too expensive to use such an item to consistently have, for example, furioso fists.

If the enhancement bonus is desired, an alternative is finding a way to cast the 4th-level Drd spell superior magic fang [trans] (SpC 136). This is a muito expensive wand (a wand of a 4th-level spell at caster level 20 costs 1,200/charge), but that 60,000 gp item grants all of the creature's natural attacks a +5 enhancement bonus and, by the time such a wand can be afforded, the character probably won't be participating in 50 more fights during his career.² Note that this also assumes the spell superior magic fang can affect unarmed strikes. (While the spell presa mágica implies that spell can, the spell superior magic fang lacks such language.)

However, all these all run afoul of the opportunity cost needed to activate them, so I understand why you'd avoid them. Attempting to generate such effects mais rápido makes the price superior and makes the options below ainda mais appealing.

Let's do some math!

Amuleto dos Punhos Poderosos (DMG 246)
O amuleto de punhos poderosos specifically improves ambos the wearer's unarmed strikes e the wearer's natural attacks, but only grants both enhancement bonuses on attack rolls and damage rolls.

Amulet of Mighty Fists
   Enhancement Bonus         Cost
          +1             6,000 gp
          +2            24,000 gp
          +3            54,000 gp
          +4            96,000 gp
          +5           150,000 gp

Infelizmente, o amuleto de punhos poderosos apparently can't grant the wearer's unarmed strikes and natural weapons magic weapon special abilities (that is, at least it can't without DM permission).

Bracers of Striking (Magia de Faerûn 155-6)
These cost 1,310 gp, weigh 1 lb., occupy the bracers slot, and grant the wearer the ability to make unarmed strikes as if the wearer possessed the feat Improved Unarmed Strike (very carefully não granting the actual feat). They can be made magical as if they were a blunt duplo weapon (hence any magical enhancement bonus or special ability costs double, the text implying that each "fist" (or head, I guess?) can't be magicked separately).

Bracers of Striking        Cost
 Enhancement Bonus    (1,310 gp)+
         +1            4,000 gp
         +2           16,000 gp
         +3           36,000 gp
         +4           64,000 gp
         +5          100,000 gp
         +6          144,000 gp
         +7          196,000 gp
         +8          256,000 gp
         +9          324,000 gp
        +10          400,000 gp

The item is not as explicit as it could be (the braçadeiras can be modified with additional magic weapon special abilities but don't say those affect the wearer's unarmed strike, and I've read at least one argument wherein it was said seriously that the braçadeiras' themselves must be used to beat a fool to employ their weapon special abilities), but if one assumes the enhancement bonuses apply to unarmed strikes and one's apenas making unarmed strikes, the braçadeiras are a better deal than the amuleto de punhos poderosos.

Necklace of Natural Weapons (Espécies Selvagens 58)
This item affects but a lone natural weapon unless more cash is paid. It's acknowledge that unarmed strikes are not natural weapons (but there's a counterargument couched in the anel presas, below), so this probably doesn't do you any good, but it's included for completeness because you asked. The cost is multiplied by the number of natural weapons the amuleto affects.

Necklace of Natural Weapons  Cost to affects natural weapons numbering...
      Enhancement Bonus        One    Two    Three    Four    Five
             +1                  2      4       6        8      10
             +2                  8     16      24       32      40
             +3                 18     36      54       72      90
             +4                 32     64      96      128     160
             +5                 50    100     150      200     250
             +6                 72    144     216      288     360
             +7                 98    196     294      392     490
             +8                128    256     384      512     640
             +9                162    324     486      648     810
            +10                200    400     600      800   1,000

Costs are in 1,000s of gp. Also add to the cost
+(600 gp × the number of natural weapon affected).

Isso faz com que o colar de armas naturais a better deal than the amuleto de punhos poderosos only if the wearer has but one or two natural weapons, exceto que o colar explicitamente pode have magic weapon special abilities, and the DM must approve the idea that magic weapon special abilities can be added via o amuleto.

Outras opções
Below are a few options unmentioned by your question.

• O anel presas (Dragon Magic 101) (10,000 gp; 0 lbs.) doesn't say it can be further magicked to grant unarmed strikes enhancement bonuses or weapon special abilities, but a generous DM may allow it. Further, the anel presas implies that an unarmed strike is a natural attack, so it's useful for attempting to convince the DM that a colar de armas naturais realmente pode affect one's unarmed strike.
• O ward cestus (Guia de armas e equipamentos 6, 10) (10 gp; 4 lbs), an exotic weapon, says that attacking with it "is considered an unarmed attack," and attacks with it deal the same damage as the wielder's unarmed strike. If the DM permits the alternative class feature city brawler to fight with a ward cestus yet still be considered unarmed, this is a viable alternative.
• Just in case you later become concerned with enhancement bonuses, a Drd20 charges 600 gp to cast the 3rd-level spell maior presa mágica [trans] (PH 250) on an unarmed strike (possible according to the 1st-level Drd spell presa mágica [trans] (PH 250)). A Sor20 charges 8,500 gp to cast the 5th-level Sor/Wiz spell permanência [univ] (PH 259-60) on that maior presa mágica spell. Although vulnerable to dissipar magia effects, this combination is inexpensive enough to get done several times over the course of an adventurer's career before acquiring +5 enhancement in some other fashion is more prudent. (Costs are according to Serviços de ortografia e serviços (PH 129), but actually localizando such powerful casters is campaign-dependent.)

"So what do I buy?"

If you're using only unarmed strikes, you'll need the braçadeiras. Fortunately, some of the weapon special abilities you want only require the weapon to be wielded, so, for example, you could get the following:

• ousado (microfone 30) (+1 bonus; 0 lbs.) furioso (OA 125) (+2 bonus; 0 lbs.) fúria (HB 130) (+1 bonus; 0 lbs.) braçadeiras de golpear (Maio 155-6) (65,310 gp; 1 lbs.). Observação: O braçadeiras' description says that they "may be modified with special weapon abilities as if they were a blunt weapon." Ask the DM if that means the braçadeiras need a +1 magical enhancement bonus before they can have further magic weapon special abilities.
• +1 brash (microfone 30) (+1 bonus; 0 lbs.) furioso (OA 125) (+2 bonus; 0 lbs.) fúria (HB 130) (+1 bonus; 0 lbs.) ward cestus (AE 6, 10) (50,310 gp; 4 lbs.). Observação: As an exotic weapon, the cash saved from buying a ward cestus will probably spent overcoming the −4 penalty for nonproficiency.

And if, for example, you bind the soulmeld braços girallon (Magia de Incarnum 68) to your totem chakra and want magic weapon special abilities for that soulmeld's four claw attacks, you'll need the colar.

• fúria (HB 130) (+1 bonus; 0 lbs.) necklace of four natural weapons (SS 58) (10,400 gp; 0 lbs.). Observação: veja a braçadeiras Note, above, the colar description containing similar vague language.

Observação: A habilidade especial da arma mágica fúria (HB 130) (+1 bonus; 0 lbs.) needs to apply to cada natural weapon and unarmed strike for it to be worthwhile, yet the special ability deals but an extra +1d6 points of damage (albeit untyped) each and apenas while raging, so, really, I suggest just skipping that magic weapon special ability. The same and more goes for the weapon special ability berserker (microfone 29) (+1 bonus), e mais being that the weapon special ability is limited to two-handed weapons (which, with an inappropriately large magical ward cestus, is a possibility but a silly and impractical one).

Other Useful Items

• O garras de fera (SS 49) (9,610 gp; 1 lbs.) are a pair of + Manoplas cravadas 1 (PH 117, 118) that grant the wearer 2 claw attacks with a +1 enhancement bonus and, if the wearer already has claws, the enhancement bonus increases to +2 and the wearer's claws deal an extra +1d6 points of damage. Observação: Combined with the soulmeld braços girallon, these are pretty spiffy, but, given the item's age (and provenance!), the DM will likely update the description for a contemporary campaign. However, any DM that allows these into the campaign may also allow them to be further magicked.
• O collar of venom (BV 114) (50,000 gp; 3 lbs.) causes the wearer's natural attacks to become poisonous (Fort DC 14, primary damage 1d10 Con, secondary damage 1d10 Con).
• O demonhair shirt (dragão #356 69) (26,000 gp; 3 lbs.) grants the wearer immunity to fear, and the wearer can spend 2 rages simultaneously to enter a demonic fury that, among other effects, grants the wearer impressive bonuses that explicitly stack with rage and frenzy (!) and makes all the wearer's weapons (including natural weapons) chaotic for overcoming aligned DR.
• O gauntlets of ghost fighting (microfone 216) (4,000 gp; 1 lbs.), among other effects, causes the wearer's melee attacks to affect incorporeal creatures normally.
• O ghoul gauntlets (microfone 104-5) (10,000 gp; 1 lbs.) grant the wearer, before making an attack, the ability, once per round, to declare one natural or unarmed attack as capable of paralyzing a hit foe (Fort DC 13 negates).
• O ring of adamantine touch (microfone 121) (6,000 gp; 0 lbs.) causes the wearers weapons (including natural weapons) to overcome DR X/adamantine as if they were adamantine. Given its rarity, this is likely a late-game purchase if at all.
• O tentacle extension (illithid) (E 75) (32,000 gp; 2 lbs.) grant a lone tentacle a +2 enhancement bonus on attack and damage rolls, a 1-step size increase to damage, and an extra 5 ft. of reach. Observação: O soulmeld displacer mantle (Magia de Incarnum 64) bound to your totem chakra grants 2 tentacle attacks. However, that doesn't make this a good deal.
• O wyrmfang amulet (microfone 148) (1,350 gp; 0 lbs.) causes the wearer's natural attacks and unarmed strikes to overcome DR X/magic as if they were magic weapons. Given how frequently this arises, this is likely an early-game purchase, later sold when better options become available.

¹ Fortunately, the infusion personal weapon augmentation targets but a arma and its description only mandates it affect a arma, saying nothing about the weapon being manufactured, so affecting an unarmed strike should be a thing even if not a monk.
² This overstates things a little. The character'll probably need to buy two or even three eventually. Each wand contains sufficient charges to use 1 charge per encounter for about 3 levels worth of encounters. A third wand tips things in favor of the amuleto over the course of the character's career.

Eu recomendo anel presas e colar de ataques naturais.

• O anel presas gives Improved Unarmed Strike, Improved Natural Attack (unarmed strike), and bonus Con damage on a critical hit. You don’t need Improved Unarmed Strike, but Improved Natural Attack is pretty nice, as is the Con damage.

  Dragon Magic—10,000 gp

• O colar de ataques naturais can get special weapon properties, unlike the amuleto de punhos poderosos, and for fewer than three natural attacks, the colar de ataques naturais is cheaper.

  O braçadeiras de golpear are only a little more expensive if you are using Two-Weapon Fighting (110 gp), and free up a ring slot which may be worthwhile.

  Espécies Selvagens—For each natural weapon affected, 600 gp plus the cost of the weapon properties applied. For TWF, even unarmed, I would presume that to be 2×(600 + bônus² × 2,000 gp + diverso) gp.

Aside from these, I’d probably focus on getting as big as possible. That will benefit all modes of attack.

Monks have much better based unarmed damage. While gaining levels as monks might not be an options, there are items that help "play pretend", see this GiantitP thread:

• Gauntlets of the Talon (Relic), +5 Monk Level (or 1d8), 20,000 gp (Complete Divine, p. 97)
• Monk's Belt, +5 Monk Level (also gain AC), 13,000 gp, (Dungeon Master's Guide)
• Monk's Tattoo, +4 Monk Level (restricted to monks), 80,000 gp (Magic of Faerun, pg 163)

Daqueles, Cinto do Monge is an obvious choice, both cost efficient and granting a bonus +1 AC when unarmored.

Also, as mentioned by KRyan, increasing size is a very effective way of gaining more damage:

• the character's Strength increases
• the weapon/unarmed damage die increases

As a result, any items that allow growth can be worth your while:

• a Wand of Enlarge Person only costs 750 gp for 50 charges for +1 size category, and a friendly Sorcerer or Wizard can easily use it on you
• Expansão or Giant Size are other options, but I could not find how to get them in an item efficiently (a Dorje of Expansion can only attain Manifester Level 6 (1+5) when it would take ML 7 to get a +2 size categories and Giant Size is a 7th-level spell so you would need a rod or scepter); friendly casters are still an option of course

In the same vein (and although not strictly on topic), I would advise nabbing Powerful Build if possible as it increases your effective size by 1 size category, however I only know of Golias (Races of Stone) and Half-Giant (Expanded Psionics Handbook) to have this racial trait and know of no other way to get it. Of course, Golias are pretty good barbarians, so it could mesh well with your concept.

A Golias under the effect of an augmented Expansão seria Enorme and count as Gigantesco for the purpose of computing the damage it deals with Ataques desarmados.

Dice and average damage (per unarmed attack) dealt by various combinations.

Tamanho médio

• Regular Médio character: 1d3 => 2
• Golias Médio character: 1d4 => 2.5
• Monge 1 Médio character: 1d6 => 3.5
• Goliath/Monk 1 Médio character: 1d8 => 4.5
• Monge 5 Médio character: 1d8 => 4.5
• Goliath/Monk 5 Médio character: 2d6 => 7

Tamanho grande

• Regular grande character: 1d4 +1 (Str) => 3.5
• Golias grande character: 1d6 +1 (Str) => 4.5
• Monge 1 grande character: 1d8 +1 (Str) => 5.5
• Goliath/Monk 1 grande character: 2d6 +1 (Str) => 8
• Monge 5 grande character: 2d6 +1 (Str) => 8
• Goliath/Monk 5 grande character: 3d6 +1 (Str) => 11.5

Tamanho enorme

• Regular Enorme character: 1d6 +2 (Str) => 5.5
• Golias Enorme character: 1d8 +2 (Str) => 6.5
• Monge 1 Enorme character: 2d6 +2 (Str) => 9
• Goliath/Monk 1 Enorme character: 3d6 +2 (Str) => 12.5
• Monge 5 Enorme character: 3d6 +2 (Str) => 12.5
• Goliath/Monk 5 Enorme character: 4d6 +2 (Str) => 16

Note: I just saw your comment about having to recompute statistics when changing size. I would expect a character built around this concept to either maintain several character sheets (1 per size) or to have a character sheet annotated with all the gains/losses in the appropriate sections; obviously recomputing one's character sheet mid-game is a big no-no.