Have you though of making your own? There has been a couple good threads in the last year on both the Miniture engine and Radial and rotary enging groups on Yahoo. Basicly a good piece of 1/2" drill rod for the stock and a lathe with a collet to handle the drill rod. Cutting the stems is easier if you make your final pass a very deep cut. You have more stock to support the stem that way. Then you are left with the project of groving the little stems, but a good collet in the lathe will make life easier. There is a guy that makes valves for the 1/6th scale P&W Wasp engine. They are a bit smaller than what you are asking for though. 3/8" head size and overall length of 20.3mm with a stem Diameter of 3/32". Check on the R_&_R_Engines group, in the files area and you can find some soruces for haveing valves, springs, and the like made.

The other group is Min_Int_Comb_Eng Also a good source of info.

Don