Schrader taps and dies are available, but they're generally not cheap, as nothing else uses that size thread.
I needed to make a screw on adapter for one once and ended up buying a set of metal valve caps from a cheapy motor factors, then drilling a hole in the end for the pipe and gluing in a bit of plastic to depress the valve core.
cheers for that have rung eldest boy to go down place that sells engineering supplies to try and get one.
hopefully they will have one and he can bring with him tonight.
The internal thread of a schrader valve as stated is a one off: well not quite as there are 3 sizes
The taps are mega bucks, I bought 3 about 12yrs ago, they were well over 50quid each. Bought them from a sister company of tracy tools, give them a call