The old style shot bags made of material rather than the new shot bags. You can use the newer bags but you need a plastic liner inside such as small bathroom trash bags as sand can seep out thru the mesh.
These aren't free, but they aren't exactly expensive either. They are strong, nice and clean(with no lead residue). We bought a bag of dry playground sand (kept indooors) at Menards. We filled them, then sewed and Gorilla glued the ends shut. They have been really helpful. Good luck.