Besides Craigslist, check out ebay also. I got two bikes there and I wouldn't have a problem getting another one if I so desired. You get a cheap frame from Walmart you'll find that you can't upgrade the parts if you wanted to. A lot of those bikes can't take certain quality parts. You get yourself a decent mid-level frame and you can put any new part on it that you'd want. If it were me. I'd put all of my attention into getting a good high quality frame. And then over the years gradually upgrade all of the components. After about five years you'd have yourself a really nice bike. Keeping everything under $200. You're not going to have much. Go to ebay and see what they have for under $300. You'd be surprised.