Try Craig's list, that's where I found this one... and I saw lots of other interesting stuff on there. You have to check it regularly though, stuff changes a lot day by day. Depends on your price range though, for me convenience was more important than price (found online, paid $75). If you want to pay $10 or whatever you'll probably have to do the footwork (yard sales etc). Almost no one bothers to put $10 bikes online.