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  Where to Buy All Things Atari
Obviously thrift stores, estate sales, auctions, and grandma's closet are the best places to find games real cheap.  But it's getting hard to find Atari stuff that way these days.  I recommend the following:  
Does AtariGuide buy and sell games?
If you've got a closet full of games, I may be interested in purchasing them from you.  Drop me a note and let me know what you've got.  As for selling games, visit:
The AtariGuide Store

Auction Sites
Bid and buy from people just like you,  from around the world.  It's alot safer that you might think!
Ebay is the by far the best source, with thousands of Atari items listed each week.
Yahoo Auctions and Amazon also have a limited selection of Atari items up for auction.

Online Stores
One-of-a-kind Sites
The AtariGuide Store stocks lots of Atari items as well as AtariGuide on CD! has the widest selection of Atari products of any internet game seller.
Hozer promotes recent releases by Hobbyists and will make a cartridge from any rom.
O'Shea's has a gazillion copies of a handful of games that have never been opened.
Sean Kelley makes a really cool Multicart - 256 games on one cartridge!
Telegames new releases of classic (mostly Mattel) games.

More Atari Retailers
Good Deal Games
Stella's Boneyard
Tomorrow's Hero's
Video 61 & Atari Sales
Video Game Connection
Video Game Exchange