Any door from a 70-81 Firebird will fit although you may have some trouble finding a decent one depending on where you are located , I would check for any old car salvage yards that may be in your area .. you can sometimes find them on line like Ebay but shipping can be costly .. if you know someone who can weld you should be able to buy just the outer "skin" and have it put on ..
I believe some restoration catalogs list skins for 70-81 firebirds , I know they are available for the camaros but im not sure for the firebirds ..