Hi-
A good place to start would be on e-bay. Do a search for mini-bike frame or any such variation and hopefully you will find something that you can purchase at a decent price and perhaps even close to where you live so as to avoid shipping costs. I have found lots of little diamonds in the rough on ebay; some are even pretty rare mini-bike models just waiting to be restored with a little TLC. As you and your brother probably have already discovered, half the fun is putting something together from scratch and finding the right part for your application. Also, do not over look Craigs List, your local flea markets, swap meets, and even garage sales. There are a lot of mini bikes waiting for someone like you and your brother to discover, purchase, and repair. Have Fun,
RacerX