I made a set for my 66 NYer years ago. I went to a roofing supply store and asked for goodyear roofing rubber (they call it membrane). A lot of commercial buildings with a flat roof use it. I asked for a couple yards thinking that would be enough. Cost about $20 in 2001. I was surprised when they opened the door and carried it out on a forklift. Turns out a couple yards is 6 ft by 30 or so feet, weighed about 40 pounds. There are a lot of uses for it in auto work. So I have a lifetime supply for car work. I am sure prices have gone up some since then, but you don't need to buy two yards either.
I originally used it for making the splash guards under the hood. Used it to line the drawers in my tool box too. When replacing the tail light gaskets I used the originals. laid them out, drew the outline on the rubber with a gray sharpie and cut them out with a xacto knife. Drilled out the screw holes and reassembled the lights. It's about 1/16 - 3/32" thick so slightly thicker than the originals.