When I started using a tumbler, I bought some corncob pet bedding from PetSmart. It was a lot cheaper than the stuff from Midway, and has worked just fine for everything .30 caliber and larger.
Now, I've cleaned some .223 brass with a very fine media that came with a second tumbler we bought used last summer. I think it's ground walnut shell, but I'm not sure, as it was not in the original packaging. It did a good job on the .223 brass, and flowed out of the shells really well.
We were at PetSmart today, looking for a particular pet food for the varmints. While there, I looked in their lizard/fish/bird sections for something a bit finer than the corncob. The only thing I found that might work was some aquarium sand. So I got to thinking, why don't we use sand as a polishing medium? Is it too abrasive?