Albion Alloys make tube that is for all intents syringe size, Comes in a variety of nesting diameters.
Nest a thin tube/wire inside a larger one to get the effect of a valve stem.....might take all of ten
minutes to make a set of 4. A little black paint, and you are done.
Alternate source: New England Small Tube............these guys will make you buy at least 2 dozen
pieces, but they also can cut to length for you, and de-burr. Other options exist, i'm sure.
Question.....why the big deal about valve stems....I can't think of any 1/18th that has been release in the past 3-5 years that has them ?.....maybe 1 or 2 TSM models, and that's it.
Also try Grand Prix models in England, they carry a TON of 'spare parts' for modelers,