According to an article by NPR, there were close to 100,000 trade associations in the US around 2010. An association specifically for automotive upholstery shops was not among them.
The last trade association for auto upholstery shops in the US was NAATS (National Association of Auto Trim Shops), founded by Nat Danas in 1953. The association’s monthly publication, Auto Trim News, featured shop profiles, technical information, business advice and directories of suppliers. NAATS also offered a Master Craftsman certification to members with a prescribed amount of experience and who vowed to abide by a code of ethics.
Danas sold Auto Trim News in 1991, and NAATS gradually faded away after years of having a strong influence on auto upholstery shops across the US. Do a Google search of you’ll find many shops who still proudly show off their certification.