Try looking at Eberron
Particularly the nation of Karrnath, where mindless undead are accepted implements of war, smarter undead have been created for yet-better warmaking, truly intelligent undead can be accepted members of society, and the Blood of Vol cult to undeath-as-immortality is a majority religion.
3.5’s Libris Mortis has a little detail on undead societies
Unfortunately, not much, but on pages 14 and 15, there are descriptions of societies where undead are “Open Members” and also of purely or dominantly “Undead Societies.” Sigil is listed as one tolerant place where undead may be open members, while a city named Nocturnus is referenced as having a Pale Ward where undead are the only legal residents. Nocturnus is mentioned a few more times in Libris Mortis, but unfortunately never in any great detail that I can find.
Fallout 3’s Underworld might be somewhat useful
It’s a city of “ghouls” (but Fallout ghouls are actually alive, just made to look hideous by radiation); might be relevant to use as an idea of what it’s like, though I don’t recall the game really going into too much detail other than “oooh, we’re spooky and different, but peaceful; how do you feel about that?” Which might work for you, but you might want a bit more.