Any decent parts place should be able to do it for you... the main agent price on it might not be all that much different, I don't have a pricelist to hand else I'd look it up ;-)
It stops muck from getting to the seal, so it's worth doing. If you are going to be in a lot of sludgy wet mud then I'd do it. Last mud I saw at Bramcote was very sandy and fairly abrasive, so anything you can do to keep it away from your seals would be a bonus !