You can probably do this in a backyard foundry. As EVILDR235 said you can melt wheels. I've heard that used pistons are the best scrap metal, you can buy this scrap pistons from a motor rebuilder, maybe $1 a pound, maybe free if you find the right deal. One of these days I want to build a oil burning foundry furnace models after Steve Chastain's tilting model. This furnace burns used motor oil, melts 100 pounds of aluminum each hour.

Steve recently published a book on how to cast custom pistons. Are these are good as the cast pistons you have buy? I think these might be pretty good. You can find these books at http://www.lindsaybks.com or Amazon.