What ksisler said, and also garage sales and swap meets/flea markets. (Check this guy out: http://www.mrmartinweb.com/bicycle.html; note one of his bikes is a Le Tour he got at a garage sale for $10! His capsule description might be useful for you)

For the very best deals, you usually have to have at least some basic wrenching skills/tools, because the very best deals are bikes that are super-low priced because something simple is wrong that the owner can't deal with. Sometimes even as simple as just pumping up the tires!