Hi Richtung,
not sure what's left of your seal, or whether anyone else with a good seal could assist.. but could it be worth taking yours off, measuring thickness and diameter and trying to source an O ring off the net? otherwise, a second hand pipe with a good enough seal, or even a new pipe complete with a new seal might be your only option. This is of course assuming.. that even years down the line you still cannot buy/source the seal separately.