Turboguzzi,
I got my valves from a supplier of Japanese valves in Thailand. I found him on ebay after doing a search for cb500 intake and exhaust valves. If you mention my screen name (same for ebay as for sohc4 site) He should remember. His ebay record was very good. He was selling valves in pairs, one intake, one exhaust, but I was able to get him to sell me a set of four exhaust at a very good price.
This is his email:
billiongain@gmail.com
hym