I bought my first Featherweight on eBay. I bid on 4 or 5 before I got the one I won. I had a max price, and that was all I was willing to pay. There are lots and lots of FWs on eBay, so you really just have to not get impatient or above your wallet.
The second FW I bought was found on Craigslist. I liked that better since I was able to try it out before I bought it.
Pam