A note on therm-flo:

Therm-flo is a modified food starch that works well in canning recipes as a thickener. It is usually mixed with water to make a paste, then added to tomato products to thicken them. If you don’t like the idea of adding starch to your product, tomato paste is another option to thicken home-canned tomato products. Therm-flo is always available at Amish or Mennonite bulk food/grocery stores, and I’m sure it’s available online, too.