The stuff is called "decking". Try SMS.
https://smsautofabrics.com/
Can't remember if it was SMS or not, but one of the places I checked back when I redid my Ranch Wagon only had decking for 55 chevy wagons. I restored mine using minimal filler, glazing putty, lots of sanding, Gravel Guard (for texture) and plastic based rattle can paint like Fusion. Didn't look exactly original but it looked fine. Same procedure on interior surface of rear wheel wells, but I had to take off original coating (kind of hard to do.)
Instead of Gravel Guard you can use SEM Chip Guard.