I like craigslist, mostly. I have an ad up for "vintage 50cc bikes wanted" and got my carabela, wards riverside 450SS, Lazer, yamaha trail master, Italjet Minarelli P6 race motor, and a bunch of obscure parts from that one ad.
Got a bunch of people trying to sell me their overpriced junk modern mopeds, too, but it's worth it.
Also just scored a rare 1962 (I think) Ducati Piuma 48 from Craig's. The sellers are in MA somewhere, and it was ten minutes from my house.
Plus I found an antique bicycle license plate for ten bucks that goes perfect with my bike.