i would...
look into trucking supply companies....
...or, if you wanna be green (or cheap) ask a local sign company for a take down. those large temp signs for businesses are tough stuff. they are made to be stretched tight and handle serious wind loads. get one, cut it to size, punch in your own eyelets, and voila! just keep the printed side down.
now, i believe due to the printing, most co's are supposed to destroy the sign when taken down, buuuut... my barber just offered me a few that could cover my house roof if i could move them. he said he gets various sizes. apparently he owns a storage warehouse, and one of his friends that hates to see waste drops them off... so, i think you could easily get one if you stopped and asked.