Rice sacks aren't the only alternative sail material available. Folks who make their own tiny boats from plywood also use common cheap tarp material for sails. The most common is the ubiquitous blue tarp sheet used for covering cars, truck tops, as shade canopies, and even temporary roofs. A second cheap sail material is Tyvek, ordinarily used to moisture seal homes before the siding is affixed. More on cheap sail materials can be found at the Puddle Duck Racer homebuilt site.
Tyvek on the left, blue tarp on the right.