Most store bought cutting boards are polypropylene, not UHMW. Not that it will matter much for a fence.
Look up plastic supply houses in your area and call them up and ask if they have a scrap bin you can look through. You can often times find scraps for pennies on the dollar this way. I've never paid full boat for plastic via this method.
The place I go to usually has a lot of HDPE (cutting board applications for restaurants) which is a lower grade material similar to UHMW but half the price and not quite as slick.
If it has a texture on, you can almost be assured that it is not UHMW.