I don't know how to get trash mats. I will say: if anyone ever made a trash tray, with a lip around the edge, I'd be quite tempted to invest.
Meanwhile, we only have the trash card, not the mat. It's possible to make do: remember cards don't need to sit entirely on it, provided they're associated with it clearly enough that you can tell the difference between the Trash and the Supply. For example, with Graverobber, we put the cards costing $3-$6 overlapping the left third of the Trash card and other cards overlapping the right third, leaving most of the word "Trash" visible.