Over a year ago there was a Dragon issue full of firearms for D&D.

Otherwise check out the firearm rules for d20modern. They mesh right in with D&D because they are d20 based, but be careful! They cause lots of damage (2d8 for most). Unlike D&D firearms they DONT avoid armour bonuses, so you should be able to use em just fine.

Treat them just like Bows and Arrows and other Ranged ammo except require an Exotic Weapon feat to use them.

I'm not sure exactly what period Ravenloft is supposed to be emulating, but early firearms are presented in d20 Past. Many of them do less damage than their modern counterparts, have only one shot, and take two full rounds to reload, thus making crossbows and swords still a very valid option.

he's right. I completly forgot about d20past.

Privateer Press has firearms rules (for Iron Kingdoms, their excellent campaign setting) available as a free download.

http://privateerpress.com/ironkingdoms/?x=rules/downloads

SpirosBlaak (http://www.misfit-studios.com/spirosblaak/spiros.html) has rules for firearms.

d20 Modern and Star Wars d20 also have "firearms like" rules. The d20 modern rules are avaliable via SRD, so take a look before you spend money on the book.

Warcraft d20 also has firearms rules. There's no special bypass rule for armor*, and firearms have a chance of exploding on a very low d20 roll (or when critically failing a save against a fire attack)

*: Note that armor *is* effective against firearms. It's just that both crossbows, longbows, and firearms are so effective armor isn't worth it on a dollar-per-dollar basis.

The Forgotten Realms setting has rules for 'smokepowder'. We used them in a game where my gnome had a blunderbuss. Overall, they were pretty fair but perhaps slightly underpowered. Smokepowder being (obviously) a representation of gunpowder, the rules there are good if you're dealing with early version firearms.

There's also some firearms of in DMG.