I used some worn out walnut media. It is lighter and works well in a rear bag for hunting as well. To help keep it water proof I put it in a seal-a -meal bag first and then put it in a shot bag with a strap and buckle so you can just strap it to your pack.
The best sand I have found for sand bags is to go find some huge red ant mounds, and use the stones that they use to cover their mounds. The stuff is pretty much perfect.
Usually use plastic pellets like they feed into injection molding systems. Won't hold water is fairly course and lighter than sand. A large bag will go a long way. Stuff below should do pretty well.