If you are in the Dallas area, Firewheel Classics has been great to me so far. They specialize in first gen Camaros, but have been really good getting me Firebird parts.

http://www.firewheelclassics.com/

She said they might start advertising they have Firebird parts too. I saw a guy in today buying a bunch of parts for a '66 Chevelle, and I heard them say to someone on the phone they do second gen too, so it looks like they don't necessarily "specialize".