Another source for feed sacks is the large bags of dog food, like the 40 pound size.
Look for the kind made of mesh material, not paper. Of course this source works only if you have a need for that dog food anyway. Or know someone who does...
I turn the sack inside out before I stuff it, so that I am NOT shooting at a picture of a dog; that would put me in the dog house around here.I stuff mine with other sacks, plastic shopping bags, plastic wrapping off freight, etc. I close it with some cord laced through holes poked with a knife, and spray paint some dots on both sides to shoot at. When it starts to leak, I just put it all into another new sack.