Hi Bryant,
I am doing the same thing, and my first choise was the gasoline brand. I choose Mobilgas (mid to late 50')which narrows the scope a bit.
I have bought a couple of books about Mobil and gas stations and always keep my eyes open to pictures on the Net. I found thebooks here at Oldgas' connection to Amazon.
This is a nice one:
http://www.amazon.com/exec/obidos/ISBN=0879389451/primarilypetroliA/ Webers in Ft Worth has many nice older photos available to look at and for sale:
http://www.weberspump.com/acatalog/Products_Nostalgic_Photos_21.html What time period are you thinking of? This also has a lot of impact, since it affects the architecture, choise of pumps, signs etc.
Good luck with your project!
Claes