Burlap is the best if you can find them. The poly-mesh bags are more likely to tear after being exposed to the sun & elements for a while (I've had problems in as little as a week).
I purchased 100 about 10 years ago. I still have them and they are 100% GTG. I keep them stored flat, in a box. They are the white, poly, 17x25 sized, 50 pound capacity. You can find them for about .30 cents each on line.
durability of the bag is key. technically you could make a sandbag out of a plastic marsh sack, its just not going to hold up very well if it gets hit by some sort of projectile. look for something that wont completely shred if hit by a projectile be it a bullet, shrapnel, etc.
With this weeks upcoming warm weather, counties that have flooding will have sandpiles and bags available for free at announced pick up points no questions asked. I work near such a site at 6 span bridge in Elkhart.