A much easier and better method is to visit your local airports and meet with the pilots, resident maintenance shops, and independent mechanics about your services. Or better, if you have friends or clients who are involved in aviation ask them if they would help you with some introductions. To actually perform the work you don't need any certification or license. The certification comes into play with what materials are used and when the new upholstery is completed and physically installed in the aircraft. For many years, when I had a customer who wanted a new interior I would remove it and take to an older gentleman in town who was a magician with all things upholstered. As the aircraft mechanic, I would handle all the FAA requirements with correct materials, etc., leaving Mr. Frank to work his magic on those tired old seats and panels. Good luck.