I am looking for a tumbler for my daughter. She likes finding rocks, and I bet she'd love polishing them up. She finds a LOT of rocks, so I'd like a bigger one than the little ones you can get at Target. I've seen some double-barrel ones that look good. Are there bad brands to stay away from? Great brands I should get no matter what the price? A good place to look for used ones? I've been checking craigslist, and not finding much. Thanks for any tips!