Reddit mentions of HDPE (High Density Polyethylene) Plastic Sheet 1/4" x 24" x 36" Black

Alright, so the going strategy in these cases is to put a normal sized hard pad ON TOP of your existing cloth pad like this (that black part is my glorious xxl pad, and the shiny part is my hard pad). So maybe consider that since it's kind of hard to find a hard mousepad, sold as a mousepad, in that size.

And if you go that route, just any 'ol hard pad from a popular manufacturer will do. Logitech, corsair, ... lots of companies make them.

But if you insist on an ultra-wide hard pad, you might use one of these options below (or something similar) as your mousepad and either deal with the length not being exactly as you wanted (36x11) or by getting one of the HDPE pads and cutting it down to the size you want:

• pvc mat
  
• leather pad
  
• HDPE sheet (smooth)
  
• HDPE sheet (textured)
  
  Note: a lot of times the edges on the HDPE sheets are sharp enough to annoy if you lay your arm on it for a long time so you'll have to sand those down or whatever if you care about that. I can use my HDPE pad without issue though, the edges don't bother me enough to fix them lol.