might get lucky if you check kijiji often but unlikely you'll find exactly what you're looking for:
http://calgary.kijiji.ca/c-buy-and-s...AdIdZ153804628
You also may have problems finding something 15' long in general, longest I've heard of is 14'.
You could also try a place called Urban Glass, they usually have large sheets that never got used and they sell for cheap if you talk to the right guy. I think the guy I talked to previously was Mike.