The topic of valves have come up a few times. I'm not sure where (if?) people are sourcing new ones, NOS ones, etc..
The shop that sponsors our Honda can make any one-off stainless valve for $75. In quantity (25+) this price can come down a fair amount.
There are several items that make these better than stock in terms of performance. (You won't be able to feel the difference, but it does add up.)
And you have the benefit of knowing that:
1) They are the same type we've set records with in Bonneville and El Mirage - with no valve failures. (Mine are slightly larger, but same style/materials.)
2) The person making the valves is very familiar with the Honda engine. They'd come from a specialty machine shop that works mostly on small overhead cam engines.
If you've got interest, PM me, or go to enginedynamics.com and use their web form to request more information on getting a part made.
(Please do try to be considerate of Mike's time. The "tire kickers" take away from his time for the paying customers . . . )