Pontiac Historical Services (PHS) will have the full documentation for your car. Can replicate the build sheet information, list colors, options, build date, etc. http://www.phs-online.com

Sheet metal wise, Dynacorn, AMD, and Goodmark are the most common. All have some fitment issues, so I'd say one is not better than the others. (just google them) Also, Ames performance is a really good Pontiac parts supplier. Some members use Classic Industries, but they do try to pass off a lot of 'Maro parts and being firebird. There are some obvious ones, and some subtle differences in others. Depends on how anal you are about that stuff. (I am)

If you do a search for floor repair or rear quarters, you will find a lot of threads and leads.

Or just visit Harold's site for a start to finish project with a billion pictures. (and merchandise)

http://www.firebird400ho.com

A lot of members here have extra parts for sale, or you can post in the "parts wanted" section. NOS parts are getting hard to find and command top $$.

Good luck, and welcome to the site!