Source for white foam under rubber floor mat?

[saxart]

When I removed the original rubber floor mat from my '70 C-10, there was a thin white foam between the rubber and the steel floor pans. The original floor pans of the truck are in relatively decent condition.

I recently had to remove the replacement rubber floor mat to get at the transmission hump and noticed that the floor pans (original paint) have some very light bright orange rust under that mat even though I don't drive the truck in snow or rain. I'm guessing the mat needs that thin white foam to help it breath.

Does anybody know of a source for this product?