The bags I used are feed bags that horse and animal feed was sold in. Forum member Greg White has several horses and had a stack of these bags.
I'm sure they are commercially available, but why not recycle them, get in touch with a local feed store, and find out what their customers do with the used bags. I'll bet each farm has a stack of free woven poly-bags.
The sandbag bags I have seen are pretty small, and would only hold about 20# or so of coal.
BTW: My bagger is only $29.95 + S&H, just paypal me the cash, and I'll send out a set of hand-drawn plans, so you can do it yourself!!