Part of finding reference material is knowing the subject. If you make your internet searches too specific you'll get fewer results (generally speaking). What I mean is, you're looking for a Formula Firebird .... but really, they aren't much different from any other Firebird from those years. I don't mean special editions of cars aren't in fact special, just that they were an "option" to an existing base model. So the part where knowing your subject comes in to play is knowing HOW the car was different from the base model. Books are valuable for this but also enthusiast web sites and forums. They may not offer great images but you will know without a doubt what exactly makes a Formula Firebird different from the base model. And chances are that you can put Firebird and the year into your search and get a million results that will work for you .... then you can add in the mods. I don't think it is ever a good idea to work from an artists design. They are great for generating ideas and inspiration, but knowing your subject is the first step for me. When trolling the net two things I do are, save everything you like, and try broad and unusual search terms. You never know when or where you'll find something cool. Just putting in 73 Firebird is the first in many searches, imo. You might find a really cool image of a Firebird while searching for Firebird wheels or custom paint, or flaming chicken

... one that you never saw while searching for 73 Firebird.