When I think of Fanged Four fic, I think of three writers: Peasant, Josey and Tania. At the height of the shows' popularity, they produced a prodigiuos amount of historical fiction. None of them is currently writing Jossverse fic, but their web sites are still around for us to enjoy.
If you've been around fandom for a while, you've probably read all these stories. But if you're new-ish, they might be new to you. Check them out.
Peasant's Plot, fanfic by Peasant
La Passion du Sanguinaire, fanfic by Josey
Also Comes in Strange, fanfic by Tania
You can also find a smattering of historical fic at
Slashing the Angel, Kita's awesome Angel-centric archive. And, as the mods mentioned,
http://www.fangedfour.com/ is the place to start for all your historical Fanged Four fiction searches.
Happy reading!