Hi Lance.
Is this seal the big rubbery seal that goes around the two parts of the filter assembly? Or the thin self adhesive foam gasket that goes around the very outer edge of the filter unit and recovery bucket?
If it's the former,you may have to get creative as they are very much not available, or be lucky and find a whole filter assembly. Moorgoods on ebay might be able to help, they've had a few vax 121s in for breaking and came up trumps with some Goblin Rio parts for me!
If it's the latter then they are still available as a pattern part, they were on Quаltex's systems a few years ago, so mvacs may be able to look it up for you.
Do you have the bag holder and a papee/cloth bag that fits into that with a rubber band, or the drop in paper bags? If it's the former, you can make Numatic hepaflo bags fit quite happily which almost make the original filter redundant, then just seal the perimeter of the filter unit externally with tape... Will last until the bags full anyway!
Sam