Yes I did mention time you want to spend before buying. It's not just "Shopping around"; you actually need to become a semi-expert to recognize a good deal.

You could try FreeCycle. Or find a bike shop that carries used. Find a chop shop that has junk-outs, give 'em $100, grab a frame and parts and start building. There's lots of stuff, but you need your own know-how to not get scammed.