One resource that I find very helpfull is a copy of "The Official Railway Equipment Register". You can usually find them at swapmeets. You'll want one that was issued around the time frame that you are modelling. I picked up a January 1982 copy for $3.00 at a local swapmeet. In it you will find a listing of all reporting marks and who they belong to.
If you know someone who has one, check it out.
Dan