I found the bike I'm currently converting at a thrift store for 5 dollars. I would say start there, maybe some garage sales etc. You can get a wheelset for about 115 shipped off ebay, probably less if you are a little picky. It seems to me the cheapest bikes i find at thrift stores are the ones with shot tires. It might be because they see that they are "unrideable" with effed tires, and they mark em down. Old road frames are cool, though I would look for something that has an american bottom cracket, as finding foreign parts can be discouraging if you ever want to replace the BB, etc.