Webley & Scott Branded Shotgun Slip

bought it for my son, but had a chance to do some plinking with it and it is a wonderful rifle to shoot. He was absolutely amazed at the accuracy. But now the Piston Seal or cup/ring (?) has been damaged by likely some dirt that got in the barrel air intake at some point - boys...
Explains why the shots were all over the place, and not penetrating the plywood target as before. Problem is damage to the Piston Seal/Ring (made of urethane or some other hard plastic) due to dirt build up over years.

Having a great deal of difficulty trying to find the replacement part for VMX .22- 5.5cal Piston Cup/Seal/O-Ring. Please, where can one purchase a replacement Piston Seal?
Still a lot of life left in this wonderful shooter.

internet isn't what it use to be...