On the gas engine the fuel filter is serviced by Volkswagen as the whole housing with the filter pre installed. I seen the filter serviced separately before, I bought one on ebay from a seller but he doesn't have the orings for sale or a supplier to get from. So I'm hopping someone else might know somewhere
Go to any local hydraulics repair/service shop with the o-ring and they can measure it and supply one. You may have a hydraulics seal distributor near you (don't know your location other than "United States").