Hey Mitch. IMHO I would start with places like National Parts Depot(NPD).These people are extremely knowledgeable about 1st generation Mustangs. And get you a copy of Mustang Monthly,or Hemmings Motor News, or the like. They are always advertising parts vendors by the gross.
Also, get involved with a regional Mustang Club of the MCA and become a member with their club. They are lots of knowledgeable people on this forum that can help you as well.