so you have a few choices, if its goosed you can no doubt get it repaired professionally (Hagon?) or indeed have a go yourself but you would need something to compress the spring and how will you get the seal out, is it a pressurised item? has it also lost pressure? i'm just guessing, no real experience of this myself, my DT has still got the original 30 year old shock on board
I take it you tried ebay...what Triumph model is it?
will another shock do,,,why not contact sellers of other shocks and badger them for dimensions? i reckon that might be a good way to pursue this.