where to find used bikes?

Where would you all recommend that I look when doing for a used bike? My current bike was free, and is too big for me. I want to get a basic frame set as cheaply as I can, and build it up as a project bike. It's partly for the fun of the project, and partly because I have almost no money to spend, and I can invest in components one at a time. Buying a whole bike is just out of reach for me right now