David,
I would take this problem to an engine machinist who is familiar with TRs. I'm not sure who might have these specific plugs in stock, but if you look online for Triumph parts specialists you may find someone. It's just possible the machinist will have a source as well.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon