Peter Dyson has them.
Don't forget to order the bronze spacing washer at the same time in case you don't already have one installed, and always replace it when you renew the nipple. Put a teeny dab of 'Copperslip' on the threads before you tighten it up and that makes for an easy removal when necessary. I put a Treso, American-made, version of the nipple on mine about twenty years ago and it's still as good as new.
tac