Sometimes we use round rubber ones that we pickup at the local 4 x 4 shop. They use them on Jeeps between the body and frame and they are about 2 inches in diameter x 3/4 inch thick, and they have about a 1/2 inch hole through the center. Very cheap too, like $ 1-$2 each.
Sometimes I just use those rubber fender washers that come in various diameters and hole sizes. Even ACE hardware has a drawer with them in it.
Don