I have friends who have these rock tumblers: http://www.lortone.com/tumblers.html
and use it for sanding polymer clay.
I REALLY HATE to sand, and I wonder where I could find some rock tumbler cheaper, and if you can give me good references about your own tumbler.
Lortone tumblers are worth the money. Kiddie tumblers can leak or just give out - they aren't meant to stand up to hours of tumbling, non-stop over a week. The best price I've found is at Connie Fox's site: http://www.jatayu.com
You'll need to make an insert for the tumbler, per Desiree McCrorey's instructions. Diane has already provided the link to you.