I tried eBay, but wasn’t thrilled with what I found.
T-Mobile (formerly Voicestream) and Cingular are the major GSM carriers in the US. ATT recently converted to the GSM protocol. I would ask friends and family about former Voicestream/Cingular customers–probably one of them has an unused phone sitting around.
Locally, you can find new (slightly outdated models) mobiles cheaply. A Nokia 3310 can be had for ~USD60. The downside is you have to pay a lot for the airfare 
Make sure the phone is either for your carrier or unlocked for all carriers. If you buy a phone that’s locked for, say, Cingular, and you have ATT, you’re going to have to go through the trouble of unlocking it, which usually involves paying someone to do it for you.
I got my Cingular serviced Motorola C331g phone from the Cingular website for only $29. I called to make sure it wasn’t a typo because Best Buy had the old model for $70 after rebates. You do have to sign a contract with Cingular but it’s a great phone.