Like others mentioned check craigslist or yardsales. I think the early 90's mountain bikes (fully rigid) are a good place to start. If you can bring a friend who is knowledgeable is a good idea. You might also want to check out your local book store or browse online about bike maintenance. Many of them have a section on what to look for when purchasing a used bike. More importantly test ride as many bikes as you can. Maybe even take notes on items such as "felt stretched out or felt cramped, didn't shift smoothly, felt heavy or slow, etc...". This will give you a frame of reference when looking at other bikes. If you tell us more about your height, inseam (not pant), weight, and area you are looking at some of us maybe able to help more.