At the supermarket down on da bayou you can buy rice that comes in a cloth bag. Don't remember if it's a 3 or 5 lb bag. It works great as a shooting sandbag, just don't get it wet.
It's possible that a local sporting goods store/gun shop might have some. I bought some last year at one of the larger sporting goods in the area. They were made out of a heavy material and were just tied up at the end instead of sewn up. If I recall correctly, they were only like $3 apiece. Too bad the place is in Monroe, a little too far to drive for them. Good luck
If you are going to make your own, go to the DS wallyworld and go to the very back wall near the fabrics area and ask for the bean bag chair refill. it is Styrofoam balls about the size of a green pea which worked well for me. it compacts in the bag for a firm rest and is a lot lighter than sand and doesn't leak out of little holes making a mess like sand. The item was actually on a bottom shelf on the back wall about mid way of the store IIRC.