Slant Six "Engine Builder" valves

You probably already know this, but RockAuto does carry a fair selection of rebuilder's valves- on the home page click on the "Tools & Universal Parts" tab, scroll down to "Engine", then to "Rebuilder's Valves" and then select Intake or Exhaust valves. Takes a bit of hunting, but they're all listed by stem diameter, head size and overall length.
Good luck!