I got a tumbler for Christmas and just wanted to add some information to this thread. I'm not a reloader and found that the two gunshops in my area didn't carry media. Normally I'd be content with giving Midway some business due to their glowing reviews, but I wanted to try my new toy out immediately.

I found that you can get the media in pet sections or pet stores. Here are some links from the manufacturers. These brands were carried at Wal-Mart and Pet Smart by me:

Hartz corn cob bedding
Kaytee KayCob (corn cob) bedding
Kaytee walnut bedding
Zilla walnut reptile bedding

I didn't see any specific mentions of them on this thread but when I searched for the products I found a lot of positive reviews on gun forums as being inexpensive and effective media. I have the Zilla walnut and Hartz corn cob (my local stores only had the 25lb. Kaytee stuff) and while I've only done two razors, I'm happy with the results so far. HTH!