Yes the roof wouldn't support itself at 15ft. You would have to do something with supports. They wouldn't have to be thick supports because they only have to hold up the roof, but you would need a few.
You could probably get some custom made roof panels that were 9ft long as you can get wood in these lengths. Or if you had normal 6ft panels you could use 9ft long pieces of timber to support the roof without resorting to stilts, but the run would need to be only 9ft wide.
That is a lot of money, you could make them a lot cheaper yourself or maybe get someone local to make them. Also have you checked eBay?