Let me know if you find anything... I am located in Champaign and also wanted to avoid the high cost of shipping plywood. I ended up building two of Nick's S&Gs from $7/sheet 1/4" luan (I also considered 1/8" doorskins) - either is available almost anywhere.

I have been paddling the boats for a year and have had no problems. I sheathed the entire outside in 6oz cloth and epoxy, and filleted and taped the inside seams. It ended up heavy for a S&G (around 57 pounds), the 1/4" material accounted for about 5-8 pounds of the increase over usual weight. I would like to find a local source of better material as well for future projects, but in a pinch, fill voids and cheaper stuff should be OK.