Sources for fabric:
All these places also sell hardware (buckles, etc) and webbing.
For stiffening a rolltop closure, I've used scraps of plastic strapping, like the kind used to hold rolls of fiberglass insulation and other building materials together. Free from the dumpster at the lumber yard. Sew a tube around the top of the bag, either by folding over the bag material, or sewing on webbing or grosgrain ribbon. Insert two lengths of plastic, and maybe bar tack them in place. In fact, the webbing alone may be all the stiffening you need.
Plenty of other sources of cheap, thin, light plastic as well.