Well, I don't know where you are, I'm in Oregon (as if you couldn't figure that out) so I tried to stay local. http://www.interstatemetals.com/metal-fabrication-catalog.htm
is a company that specializes in replacement siding for existing trailers. Something you might consider is putting several pieces on your sides. Eight feet tall will have lots of "Oil-Canning" if you use a single sheet.
Yep, it will cost you. Fiberglass sheets are another consideration for you, metal will sweat so you will have to seal the wood under it.
But when you sit back on look at all the fun you are having, its all worth it.