I've had incredible luck with antelope rawhide. It's almost as thin as a sheet of typing paper and tougher than...erm, uh, well...rawhide!
There is a guy in the classified ads in PA that sells antelope rawhide strips, his name is Michael Foltmer and is out of Evans Colorado. I think he gets something like $20 for a pair of strips, but he'll send you a whole speedgoat hide for about $37 and you can back about 5 bows. The goat hide I got was as big as some of the mule deer hides I get here in South Dakota!