I used to get it from a plastics place, several around me sold it but I always ended up at a place on the old aerodrome on Purley Way.
I made carb spacers from it, I should remember the thickness as I have bits of it in my scrap cabinet, but can't.
We had similar guides at Longbridge on some of the spot welding fixtures, they used a hard wearing white plastic type material, Tufnol would burn/char and flake away with the heat.