I bet you could get recycled cat litter at a really good price. Sorry couldn't resist.I fill mine with cat litter. The cheapest I can find.
I made one to fit the longer bipods (I used the one Len sells on here as a rough pattern) and filled it with poly beads. lighter than sand and adjust with less effort.Len has a great little rear bag here in the LRH store, another one is the 4x6 red tac bag that Sean at Defensive Edge has.
Mike