The closest thing there is to an American Garter Snake Association is the American Society of Ichthyologists and Herpetologists, as far as I know. It's made of scientists who either study fish and herps or use fish and herps to study various phenomena. There is a pretty fair amount of garter snake research that's done and presented, but this organization caters to scientists... not hobbiests. In my opinion, there is a niche that could be filled here: perhaps in collaboration with the EGSA. Then, garters could be shipped to Asians, Australians, Middle Easterners, Africans, etc. to form a larger organization...
but we'd need a catchy name for it
.