How do you get 5' sidewalls?

I really want the extra height and proportions of a 5 x 10 side. However, there is no such thing as 5 x 10 plywood. (If you can find me some, I'll give ya' a dollar!) So, how do you do the side wall, without doing a full built up frame. Tongue and groove two sheets together - one on top of the other. That's just asking for the wobble. Any ideas?