De que maneiras (através de buffs ou itens) meu ladino pode causar danos radiantes?

There are a few radiant options open to your party.

1. Moonbeam, a 2nd level Druid spell, is available to the party Druid. It lasts for a minute, concentration, so it's a good way to keep the Radiant damage going without spending a spell slot every turn.
2. Guardiões do Espírito, a 3rd level Cleric spell, is another spell that deals radiant damage repeatedly if your Cleric can maintain concentration.
3. Procure por um Lâmina do Sol for yourself or the Eldritch Knight. It's a rare magic sword with the finesse property that deals Radiant damage.
4. A Maça da Ruptura, another rare magic weapon, is also an option. Its only drawback is that it only deals radiant damage to undead and fiends. If you ever need radiant damage against a creature of another type, you'll be out of luck.
5. A Zealot Barbarian (from Xanathar's Guide to Everything) deals additional radiant or necrotic damage on the first attack they hit with on each of their turns starting at level 3. This may not work for your party, as it would require some multiclassing.

6. The Paladin spells (also available to War Domain Clerics) Favor Divino (1st level) and Crusader's Mantle (3rd level) add Radiant damage to your weapon attacks and the weapon attacks of all allies within 30 feet of you, respectively. They each last one minute and require concentration. These are similar to a "lesser Holy Weapon", but may not be easily accessible to your party.

   Também é importante notar que o Luz do dia spell that you mentioned ironically doesn't create sunlight (see a esta pergunta), nor does it deal radiant damage. It wouldn't actually be able to help you with a vampire. If the spell did create sunlight, it would say so, as in the 6th level spell Raio de sol.

Itens Mágicos

A Maça da Ruptura ou um Lâmina do Sol are your best bets. Both are rare items, so they're generally considered acceptable for tier 2 characters. There are other items that come up when searching for "radiant" and "damage" in D&D Beyond, but they're legendary or setting-specific.

O Lâmina do Sol is the best choice for a rogue, as long as you don't mind the fact that walking around with what is effectively a lightsaber makes hiding impossible.

While the blade exists, this magic longsword has the finesse property. If you are proficient with shortswords or longswords, you are proficient with the sun blade.

You gain a +2 bonus to attack and damage rolls made with this weapon, which deals radiant damage instead of slashing damage. When you hit an undead with it, that target takes an extra 1d8 radiant damage.

O propriedade da fineza means you can Ataque furtivo with it, and because the weapon's base damage is radiant, the entire damage from the sneak attack is radiant, too.

Outside Assistance

Unfortunately, your party doesn't have anybody who can cast Crusader's Mantle. It would be available relatively soon to a War Cleric, or a little bit later to a Paladin. It adds a small amount of radiant damage (1d4) to every successful weapon attack made by a person within 30' of the caster.

Outras opções

Unfortunately, there really aren't any in D&D5E. Previous editions had a lot more bits and pieces that have not made an appearance in 5E. That said, I can't recall any that would have helped anyway.