A Mace of Disruption or a Sun Blade 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.
The Sun Blade 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.
The finesse property means you can Sneak Attack with it, and because the weapon's base damage is radiant, the entire damage from the sneak attack is radiant, too.
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.
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.