Bill and/or Terry are the best bet for hard to find original parts.

D&R is a Camaro vendor but they dabble in some Firebird parts. They carry sheet metal and for those who visit the Carlisle show, or other shows they attend, they bring a lot of panels, including 400 hoods and complete floors. Shows are the best way to buy panels because of shipping costs and shipping damage.

Rock Auto sells mechanical parts, like bearings, brushes, suspension parts, alternators, starters and that sort of thing. They carry a full line of Moog. They carry many parts, such as control arm bushings in Delco.

I forgot to mention, Pacific Auto Warehouse. They sell engine parts. I've only bought a balancer and some lifters from them. Their prices are pretty good. http://www.pawengineparts.com/

There's a few others,

http://www.performancedepot.com/
http://www.summitracing.com/

For springs, go to the source,
http://www.eatonsprings.com/