Here are a few vendors that have internet presences. You can browse their web pages, or send them email about prices, etc. Personally, I try to patronize(?) my local video game stores as much as possible, which means that I've probably never had any contact with any of these stores, so check them out first.

If you have any to add, please send them to me.

Beyond Games
Bits of Fun
Demand Systems
Game Dude
Toad Computers
Webmaster Games
Video Game Liquidators
United Games