When I was faced with this problem on our GT6 I went straight to the Newton Commercial web site in the U.K. They had the webs/straps for us.
When I recently took the seat of our '72 Spitfire apart I found the webs were similarly nasty. I didn't want to invest in a proper rebuild of these seats at this time. So... I bought 10 feet of 1-1/2" wide nylon webbing and some pop rivets. I took the old straps apart one at a time, removed the original rivets, enlarged the holes to fit the pop rivets, and made new straps (top to bottom) using the hardware from the old straps. Will they work? Well, they won't stretch much so the seat will probably feel different. However, as I mentioned, this was a budget seat rebuild.