Home Depot play ground sand, very fine and heavy and cheap as in $6 bucks for a 50lb bag. Fill the bags and use the rest on your side walks, or chuck in the back of the vehicle for extra winter traction.
On the little bags that are on top of some bench rest bags I use even finer sand that can be bought in any Hobby/crafts store for around $8 bucks a 1LB. I find the finer sand works better on the top bags as it packs better, and allows for better/smoother elevation adjustments if squeezing together.