The last Harbor Freight catalog I looked at had a 3lb rock tumbler for (I think) $19.95.
I think a double-tumbler was $29.95 ...
Look at
http://www.harborfreight.com
I've bought a lot of great stuff from them.
Shipping on orders over $50 is free but, they use something like 4936th-class mail
and delivery usually takes 2-1/2 - 4-1/2 weeks.
However, you can buy a 50lb anvil for $25 - add a couple of things to make up $50 and, they
will ship it free!
If you're in a hurry and, just need a tumbler or two, you might want to pop for postage
Jon