The clarification that I think is needed is that in experimental aircraft you do not need fabric that meets the aviation standards. However, if you do want the certified and plan to use leather or sheepskin I know that
http://www.dipi.com/ supplies the certificate with their sheepskin and probably with their leather as well.
However, I think all leather and sheepskin would meet the standards.
Auto fabrics must meet the car manufacturers guidelines. Several years ago I contacted one of the rv seat cover suppliers mentioned a lot to request samples since I thought I would get the aircraft grade fabric. That particular one used auto fabric. You could contact the seat cover suppliers to see if they use aviation fabric. Most likely they would sell you fabric by the yard. If they are using auto fabric you probably would do better price wise to purchase it yourself.