I used the filling from a beat-up 80# bag and an old 40# bag I never use. Lining the inside of the bag with a layer of foam(surrounding the actual filling) is a good idea so the bag doesn't lump or dent to much. You can put sand into bags and roll the bags up.....old shirts packed tight....utility rags you can buy in bulk.....fabric remnants.....or cement if you think your tough. Just go to the local thrift shop and be creative to find a bag density that you like. My buddy actually put a 4x4 in the center of his bag and wrapped his fabric around that, and it turned out pretty good. He didn't have to account for a large portion of the inside since that 4x4 was in there.
lol, might skip on the cement but i think i will go unfilled on the bag i was going to buy, it should bring the cost down substantially on what i wanted to buy