Good luck - I know they are quite hard to find.
I thought I lost one when I was powdercoating my rims, so I went through the the catalog at my local tire store. We where able to make up a valve stem that worked from some of the OTR wheel parts. I think the wheel hole is 13/16" diameter, and they sell what they call a spud that comes in 13/16". There are also some 90 degree stems that are low profile and screw into the spud.
Then I found the one I thought I lost, so it was a waste of time... Until now.
It's definitely a project, but they can be made yourself if you need one bad enough... Let me know if you need more info.