Your best bet would be ebay, I'd say, but the sidekick 3 hovers around the $300 mark.
OH, definitely try Craig's List. My friend came across a pretty good deal for a brand new ps3. If you call and a black guy picks up the phone and says he has a new sidekick, he's not lying. He stole it, but he will tell you his friend "gave" it to him in exchange for a interstate ride.