You need a source? Hmm... well, I tend to think "what will the PCs want to meet" and "what are the different sorts of cities out there?" Then for each city (pirate city, fey city, drow city, etc.) think up the various different types of NPCs to meet there (pirate innkeeper dwarf, etc.)
However, I do take ideas from other settings and go over them. Great stuff, y'know? Inspirational reading. However, depends on your playstyle if you'll use them the way they're written (like, look up the building of number 42 on the map).
suggestions:
Necromancer Game's City State of the Invincible Overlord. it's 3.x, but there are few enough stats of any kind. You get piles of locations detailed. Like, almost every location in the city, feels like. OCD, but it's ready for you.
Also, I believe they have a number of ok products for this sort of thing. The Lost City of Barakus has a town and principality. City of Brass I havn't seen. However, their Wilderness Guide is an update of old school Judges' Guild stuff. It's massive, and fits in with the City State. Same deal, but a massive amount of stuff. Read it in your spare time.
WOTC: Waterdeep: city of splendors; other FR products have lots of cities in them, though not as detailed. the FRCS is a great place to go. If you're in Waterdeep, might as well check out Undermountain; classic "dungeon under the city" place.
I remember something about the Greyhawk books that came out, the classic modules; I think exploration of Castle Greyhawk, but I don't know how that went.
PAIZO: the pathfinder series has some great settings in it for 3.x. Riddleport, Korvosa, etc. I believe they have settings books out for each of them, or at least for the adventure path companion books. Good to look into.