Rubber piston on your crosman bb gun need replacement parts?

I assume that you are asking about a .177 caliber BB gun not the toy 6mm plastic BB gun. You didn't state the model of the CROSMAN BB gun. If it's still in production you can get the parts from Crosman.com If it's out of production I suggest that you look on the web under Airgun repair and find a shop close to you to do the repairs.