Non-slip plywood :: Winwood Products
I'm sure there are several suppliers/manufacturers. The problem you are going to run into as being an end consumer will be minimum orders.
Variations of that product are used in lots of different industries. The key will be finding one a reasonable distance from you that will kick you down a few sheets.
Maybe try a specialty lumber yard. They may be able to order a few sheets. Or try RV manufacturers, they use that stuff a lot on ramps for toy haulers. You may have to get creative in your search.