There's generally a few on ebay: check it regularly and one will soon come up, but there's none at the moment, I've just looked.........I'm in the same boat looking out for the side trim panels for my Vitesse convertible: some hooligan cut speaker holes in them at some point...grrrr...
Failing that there are the usual suspects: Rimmers, Canleys, etc, but they'll only do you a retrim kit or a new seat, at a price!!
Convertible seats aren't that hard to come by, you just have to check ebay every few days......plus attend any local autojumbles, like I say, it's not a particularly rare bit to get hold of, and if the worst comes to the worst there's always Triumph spares day at Stoneleigh in February next year!!