There aren't many alchemical or low-magic items especially usefull against Undead. There are a few you should definitely consider:

First of all, Positoxins (Libris Mortis 74-75)

To a corporeal undead creature, a positoxin functions much like a poison, usually dealing initial and secondary ability damage or drain when the target fails a Fortitude save. The ability damage or drain dealt by most positoxins can’t reduce an undead creature’s ability score below 1. Despite their normal immunity to damage or drain to physical ability scores (Str and Dex), undead are vulnerable to the ability damage or drain caused by positoxins.

They are quite expensive. Their prices range from mere 100 GP to 1 300 GP. These may be really useful against more powerful undead like liches or vampire spawns. There are 7 different positoxins.

Ravages (Book of Exalted Deeds 34-36)

Ravages function in a manner similar to poisons, dealing ability damage or even ability drain when the target is exposed to them through inhalation, injury, or ingestion, and additional damage or other effects 1 minute after the initial exposure.

Ravages and afflictions deal damage only to evil creatures. Any evil creature takes damage equal to that listed on either the Ravage Table or the Affliction Table, plus its Charisma bonus. An evil elemental or evil undead takes an extra 1 point of damage, and an evil outsider or an evil cleric of an evil deity takes an extra 2 points of damage. If more than one kind of damage is listed, the creature's Charisma modifier and additional damage are added to each kind of damage.

There are only 2 ravages, that might be useful for this case, as the rest is way too expensive.

• Golden ice, 350 gp, Type: Injury, DC 14, Initial Damage: 1d4 Wis, Secondary Damage: 1d4 Int, 1d4 Wis

• Unicorn blood , 500 gp, Type: Injury, DC 17, Initial Damage: 1d3 Str, Secondary Damage: 1d4 Str

Special items against undead.

Aspergillum (Forgotten Realms Campaign Setting, 95)

50 GP

Each contains a reservoir that can hold up to 3 pints (three flasks) of holy water. By shaking the aspergillum as a standard action, you can sprinkle one flask of holy water on a target within melee reach. This action is a ranged touch attack (which does not provoke an attack of opportunity).

Ghostoil (Libris Mortis, 73)

50GP

When applied to a weapon, ghostoil allows it to affect incorporeal creatures normally for the next 2 rounds. Applying ghostoil to a weapon of any size is a fullround action that provokes attacks of opportunity.

Gravebane (Arms and Equipment Guide 34)

50 GP

When opened (usually by throwing it on the ground), a flask of gravebane fills a 5-foot-square area with smoke. Undead cannot enter an area filled with gravebane smoke unless they succeed on a Will save (DC 10). Gravebane ordinarily lasts for 1 minute, although strong winds may decrease this duration.

Holy Water Sprinkler (Libris Mortis, 73)

62 GP

Any hit by the holy water sprinkler also affects the target as if it had been within the splash radius of a thrown flask of holy water (1 point of damage to undead creatures and evil outsiders for holy water). A full reservoir has eight uses. Alternatively, the wielder can choose to release the entire
reservoir with a successful hit. Treat this as if the target had been hit directly by a flask of holy water, but subtract 1 point of damage for each use already dispensed from the reservoir (for instance, a reservoir only 5/8 full would deal 2d4–3 points of damage). There is no splash effect from such a hit. Filling the reservoir is a standard action that provokes attacks of opportunity.

Crucial potions and oils

Regenerate light wounds (Masters of the Wild)

50 GP

Regain 1 hp in the next 11 rounds. If the encounter finishes and you can wait a minute, it is a better alternative to cure light wounds, as the value is constant and bigger (11 vs. from 2 to 9). If you'd drink more than one, they prolong the duriation of healing, so drinking 2 potions will make you regain 1 hp for the next 22 rounds.

Hide from undead

50 GP

Undead without intelligence treat you as you have not existed. Definitely a must have.

Arma mágica

50 GP

This is crucial against vampires with damage reduction.

Other useful pieces of equipment

Antitoxina Very useful against abilities like Ghoul's stench.

Anti Disease Tonic (Dragon #331, 32)

100 GP

Grants a +5 Alchemical bonus on the drinker’s saves vs. Disease for 1 day.

Some undead creatures may cause serious diseases. This item may be a lifesaver.

Healing salve (Tome and Blood, 72)

50 GP

Applying the salve is a full-round action. One dose cures 1d8 points of damage to a living creature.

An alternative to the Cure light wounds potion, that is usable in an antimagic field.

Potion belt (Forgotten Realms Campaign Setting, 96)

1 GP

Potion Belt: This sturdy leather belt similar to a bandoleer has pockets shaped to hold potion vials and is fitted with ties or flaps to keep the potions from falling out. It holds six potions. Retrieving a potion from a potion belt is a free action once per round.

Momonga-sama covered the more standard items, I want to delve into the often overlooked items, that just require the right tactics - and often some preparation - to use.

Divide et Impera - Divide & Rule

Wood, Handaxe, Hammer & Nails in the core weapons, adventuring kit, estimated from trade goods (time, 6gp, 5SP, 5sp/lbs (?))

For a mere 10 gp, the adventuring party can have enough materials to erect a few barricades - which is very useful to either block off areas or to seal off one tunnel or another, so the group can use mass tactics against the incoming undead: by forcing the undead to pass through a 5-feet long, 5 feet wide funnel, a group of 3 can attack the single undead in the zone with only getting a single user's attacks back. if the middle one retreats one step, it is a 5 against 2 ratio.

Pá in the adventuring Kit (2gp)

With a shovel, the group can (again spending time) create more sturdy barricades from earth - or at least create difficult terrain for the incoming hordes. With a pick or axe in addition, this can be used to clear terrain for the preferred combat zone too

Tanglefoot bag in special Substances and Items (50gp)

Riot-gel... if the undead come in too fast, slow them down, glue some of them to the floor (and thus hinder the advancement of the others!). Also nice to use on creatures in the "kill zone", as it reduces the attack capabilities as well as dexterity, thus reducing the AC a bit.

Kill it with Fire!

Lamp oil & Pick in the adventuring kit. 1sp/pint, 3gp

Kill it with fire is pretty damned universal: while it might not harm the enemies too much, setting some zone ablaze does 1d3 fire damage for 2 rounds - which is not much, but if you have a smooth stone floor and a bit time to prepare the field, you can dig a grove to pour the oil into to keep the fire alive much longer as you refuel it from a barrel: let leak 1/2 pint of oil per square over the thin channel and you can keep the fire for some time... Downside is, that it is heavy and it needs preparations.

metal sheets/basins/channels estimated from trade goods (3sp/lbs(?))

With some metal sheets and a hammer to shape them, one can prepare. Or, as those are not crafters, get pre-formed flat 5x5 foot basins & channels. This makes setting up a fire-trap a manner of minutes - add in some of the nails to fix them to the ground and they can't be tossed away that easily. This also allows using the fire tactic on a soaking terrain. Also, one can easily use the basins to fortify the structures to channel the enemies - plate the wooden wall with metal sheets!

Fogo Alquimista in special substances & Items (20gp/flask)

Where lamp oil is not enough, refined Alchemist's Fire can help: it has a 5feet splash zone and deals 1d6(+1d6) direct/1 splash damage - not much, but enough to bolster up the defense. In addition, just hitting anywhere, where the basins/channels are, will ignite the oil in them, switching our fire-zone to on.

Powder Trail

Well, we try to kill em with fire already, amp it up! Let's get something actually explosive! There are rules for armas! I couldn't find my DMG at first, but now I got it again: p 145 holds guns, and so it does gunpowder!

Gunpowder: While Gunpowder burns (1 ounce consumes itself in 1 round and illuminates like a sunrod) or even explodes in the right conditions[...] Gunpowder is sold in small kegs (15 lbs capacity, 20 lbs total weicht, 250 gp each) and water resistant powder horns (2 lbs capacity and total weight, 35gp[...])

15 lbs for 250 gp a barrel. We don't need cartridges, we want barrels or bombs. Bombs detonate for 2d6 fire damage in a 5ft area, weigh a pound and can be thrown in 10 feet increments. For a mere 150 gp.

The barrel is perfect to just place around a corner, shoot at with a flaming arrow and go to cover. Or light a fuse that goes into the barrel and roll/hurl it into the undead horde! Better even: sick a flask of alchemist's fire (special substances & items, 20 gp) into the powder, and the group has an impact triggered explosive. Better get a strong person to hurl it far enough... And ask yourself how much damage 15 lbs of gunpowder do, because there are no rules for gunpowder barrel detonations. It is most likely enough to bring down a house or even the ceiling in a fortified structure: A livro indicates that 1-4 lb powder charges were used commonly for canon veuglaire, the main group seeming to be the 3-4 lb division. However, no caliber of those is given.

Not enough? We can get Dynamite! 3d6 bludgeoning, 5feet blast zone, and they are stackable... no price given though...

So, take your time and use it to make the ceiling come down in those dungeons. Crushing damage from ceilings is far than enough to rekill most things not incorporal.