Where is all this aluminum at.

I'm planning my trailer and am having trouble locating aluminum for the roof. The pieces at Home Depot are too small. I was also wondering about the glue for attaching it. Any advice will be appeciated.
Finally I'm more of a wood guy so my sides are going to be made of oak. What kind of finish last the best? I've used varathane on outdoor funiture, with some success but was wondering about epoxy finishes. Again any advise.
Finally I'm more of a wood guy so my sides are going to be made of oak. What kind of finish last the best? I've used varathane on outdoor funiture, with some success but was wondering about epoxy finishes. Again any advise.