It's really simple to make sandbags just use some green stuff epoxy putty and follow these steps. For purposes of photos and easier to see I made the sandbag bigger than it should be. Use about half as much as I did.
Cut 2 equal portions of yellow and blue.
Use water to keep your blade and fingers from sticking to the putty.
You can also use a ziplock back and put the green stuff in it to sculpt it and then take it out when it is mostly hardened and apply the texture.
Sandpaper, files, and even your fingers can make the texture for the bags.