These have never been reproduced is what I have been told by Doug Peck and Port Lines Hobbies. People have tried a number of things. They are mostly sandpaper types. I have never seen them work very well.
Your best choice will be to get another one that is in need of repair but has a good mat.
I seem to recall that the stuff is called Fish Paper. Looking it up on the net, it's described as a "rubberized fiber" mat used as an electric insulator. I have seen small sections of it in some old electronic items, and this may be what we're looking for. I haven't had time to look into it and further. I may be wrong. My wife tells me that often...
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