Sorry, I don't know of any retail source for a width greater than 8.5 ft, most likely because it's not a common size. The widest RV main body is 8.5 ft but I've seen a few models where the roof wrapped far enough down the sides to need more than 8.5 ft. Best I can think of is to put a seam near the center and use tow pieces. Sealing a seam on filon is easy enough, e.g. with Eternabond tape.