Randy,
More modern vehicles (1950s and newer) have one or more sources of interior and top "kits" for the more popular models.
For older years most folks that "restore" or have their cars restored use local upholstery shops. The shops have catalogs from suppliers that contain the materials needed. Many are duplicates or very close replacements of the original. Some are not available! SMS has a large inventory but not every one. They will if the order is large enough have material made.
In the case of your '31 Cabriolet having a local shop order material from Haartz and fabricate the top is probably your best option. Maybe SMS or another company will have kits available but you might not live long enough to see it happen.