Home Depot and Lowes both sell galvanized metal roofing sheets (as well as corrugated fiberglass versions) - They aren't particularly strong on their own, but can be reinforced with 2X4s or other wood.
Also, don't overlook bathroom paneling (both fiberglass-reinforced plastic and coated fiberboard) - stands up to water better than ordinary paneling.
Finally, most folks flock to the national building supply chains, while ignoring the local mom-and-pop lumberyards - Might not hurt to whup out your Yellow Pages, and make some phone calls...
I was looking at that stuff, I think its call Maralite ? not sure of the spelling..
thats used in a bathroom. I already made my yard debree proof as possible, and help alot of the
neighbors also to try to eliminate most of the missiles....