I don't know, I'd keep an eye out at flea markets and ask at scooter/motorcycle repair shops. Sometimes people bring them in and never return, so the shop decides to sell them after a while. Or try Ebay; they usually run about $70 for complete, rough, non-working mopeds. But do everyone a favor and if you're really luck and get a nice one for cheap, don't go ruining some original, lovely specimin of machinery. Do the dutiful thing and pass it on to someone who wants to ride.
Actually, I have an old one I was going to fix up. It's pretty complete, and I don't have time now that I'm moving to France. Where are you? I support your hands-on approach.