The shop here in Portland got them for me when they did the work, so I don't know the source without calling them. They were a step above regular stainless valves. The nitride is just a coating like they put the coatings on fork legs to make them more slippery, the 'black nitride' is a 'hard' coating.
You'll know if you will need them soon, as your clearances on your valves will beging to get tight and not last very long before they get tight again. That's the valves cupping. I took a look at mine after they pulled them out. Triumph just got a few batches of valves from their outside supplier made out of soft material. Another way to know is that if you haven't checked your valves in quite some time and your bike gets harder and harder to start, that's your valves not closing all the way (too tight of tolerances). A quick compression test will confirm it.