Rider Grip is what they are called. They stick onto your saddle and are not permanent. You can peel them off!! This type can not only add grip to the bottom of a sidesaddle that is leather instead of linen, but it can also be used as a small riser if you need a level adjustment! I ordered this, so I will let you know later what I think of it.
This is their web site if you are interested:
They also make top side grips. I would not mind getting one of these for the seat of my sidesaddle and my cut back saddle, since both are pretty slippery!