Have you tried a Jaguar dealer? They seem to keep a lot of parts in their "classic" range. They certainly had loads of stuff for my XJ40 including a full set of door seals, and the XJS is of not dissimilar vintage...
Thanks for your reply. If you do manage to track down a company to manufacture one I would be much obliged if you let me know. If I manage to track one down in the meantime I will of course let you know.