McMaster-Carr is my "go-to" place for hardware when I'm building....
http://www.mcmaster.com/If I order in the morning, I usually receive it the next day... and they have damn near everything a person needs, except beer...
I order all my stainless steel screws and hardware from them, and also all my rubber seals.
As for my FRP, it wasn't from Lowe's, but a semi-trailer place here local, I bought the FRP they put on the tops of their semi-trailers (which of course is UV resistant). Comes in 102" wide and as long as you want it, I used it for the roof on our second build, inside and out, with no underlayment... sturdy and light. It was fastened on the inside with some type of rosettes, but floated on the outside and held down by the side trim and roof vent. I also have it on the outside roof of our current build that we finished in 2008. No problems....
Hope this helps,
Mark
sid n side

"If you're going through hell, keep going......" Winston Churchhill
Side by Side - completed October 2008