Okay, well if you'd like to make it yourself, here are some tips.
Plaster can make a good modeling base since it's semi fragile, and chunks off when you scrape it with a dull edge like a hammers blade or a rock hammer, also it's relatively thin and weak when it hasn't dried fully so it's a prime time to use things like explosives, or flamethrowers on it, however I wouldn't suggest it unless you've had years and years of experience with such things like me.
As for the sandbags, you could probably make a bunch of those really easy... just go to a local fabric store, pick up some canvas or other rough material (about 1/2 a yard, or look for "scraps" they're at most fabric stores, they're really cheap usually 1/2 off the retail price which is usually less than $5 a yard unless your dealing with some really really high quality fabric like egyptian cotton, or silk. Now pick up a pack of cotton balls, you should now have enough materials to make about 200 sandbags,
Cut them into 3/4"x1 1/2" rectangles, fold in half long ways, and stitch two sides together (not the folded side, this stays that way. Now, here's the moment of decision, does it look cool like that ? or do you want it sealed ? If it looks ragtag and quickly made, and that's what you're going for, cool, leave it. If not, turn it inside out, and fill it with a cotton ball to give it a full look, then stitch the last side shut, sorry no way to close it without showing a little bit of stitch.
Also if you want to form stable walls with this technique, you can use either modeling glue, or if your fabric is stretchy enough, you can use split tooth picks (snapped part down) and make little walls by gently pushing the sandbags onto them.
Or if you want something a bit more stable go with fabric pins, flat head not balled head.