Look for "gerbil bedding". I buy that, add about an ounce of "Brasso" or liquid car polish to it, and it works very well for me. THAT SAID, it has a tendency to be a bit "large" for small-caliber (.243-ish or .264-ish) cases, meaning a case or two out of 20 will need a LITTLE extra effort to clear the inside of the case. I'm willing to do this for the 75% savings in purchase price.
I have also used pecan shells. (Similar to black walnut shells). However, if you don't break EVERY PIECE up to VERY small size, they can be a son-of-a-bachelor to get out of a small case mouth.
Regards,
Paul