the most popular brand name of this product is called AZEK. Due to popular demand, they started making one side smooth and the other side textured (like ruff lumber). This is due to homebuilder demand for a ruff texture on trim boards (this product was invented for exterior usage on home building to make wood rot extinct) Be forwarned it comes in 18' lengths only, and most places will not cut it, and it is EXPENSIVE!!!
Try a local lumber yard instead of the Big Box stores, they should have it, and may be more inclined to cut a piece for you. We sell it where I work, and we don't cut it very often, but do make an exception now and then
Scott