Often these older computers are just sitting around unused and can be had for a song or two. User groups are a good place to begin. The more local the BBS, the better. For the most part. AOL's online classifieds generally feature the most current pricing on both new and used Mac hardware.
People often can find prices lower than even the lowest street prices by shopping around online.
New computer introductions can cause used hardware values to drop by hundreds of dollars. The unnerving reality is that many people are not aware that Apple has introduced new Macs that are more powerful but sell less than the asking price for their used hardware. In the past several months such price changes have been the rule. It is a buyer's market now, after years of being a seller's market. Buying new will get you a better manual and a warranty.