Pawn shops or e-Bay:
You can find good deals these days on older non-digital SLR M-42 (Pentax screw mount) lenses, but get the best possible ones--don't cheap out. I would stick with Asahi Pentax brand and stay away from the cheaper Tamron and Tokinar type stuff. Vivitar might be okay, but it depends on the lens.
It's just hard to know if it's a good lens or not, so sticking with Asahi is safe. If you can find a really old Honeywell Pentax lens, COOL!
