If you have a lot of time, try freecycle. (freecycle.org). Here in Boston
I get beat all the time, but if it is offered 20 miles outside of Boston I
sometimes have good luck getting it.

I've picked up around 10 bikes this year either from freecycle or driving
by on trash day. Of course they aren't all candidates for SS/Fixie conversion.
Others have had much better luck.