CMSNL.COM but their prices are amongst the highest. The company has been buying out parts stocks from Honda (and the other brands) distributors and closed or de-stocking dealers for a LONG time and has a lot of otherwise unobtainable parts. They seem to know this, their prices reflect it.
Mc-Again in japan used to sell new valves at a decent price but they don't seem to have any outside distributor any more, dealing with them is a challenge unless you're Japanese is very good. And they apparently do not ship outside of Japan.