I get both crushed cob & walnut in the local discount pet store, but I've got cob from wally world too.
The cob is baged for small animal cages, while the walnut is aimed at reptile tanks.
This is totaly natural stuff, so figure on adding lots of polish to start with and a little more each time you run a batch through it.
I've been using the Flitz tumbling polish lately, but I still like the blue stuff from Dillon better.
Cheers, V