If you're looking for pro-quality stuff you'll pay for, I like the stuff from
http://www.geo-metricks.com
For freebies, you might want to search this forum for "free models" and try some of the many links that've been posted in the past (which is why I won't repost them here) I do remember that 3DCafe has some decent stuff to download, as does Avalon. Check the poly counts on them before using in a game though.
If you're a Gamespace user, there are some (IMHO) really good building textures that come as part of the software. A couple of (texture painting) brush strokes onto a basic cube or plane, and you've got GTA-style buildings.