Not sure what you're talking about as there are no valves in the carb as far as fuel and jetting is concerned, if you're talking about the fuel tube the jet screws into, I hadn't seen any sold individually... also, there are more than a few different carb options for these bikes and some use the 5mm thread size jets while others use the bigger 6mm threads. The nt carbs use the 5mm jets and the cns uses the 6mm. The Dellorto carbs use 5mm and most of their clones are 5mm but I've seen 6mm jets in some of the clones so it can be tricky to get the right jet.
What some of us do is solder the hole shut in the jet then re drill them with small drill bits, usually in the 70 to 73 drill size range. If your jet is stuck in the tube and stripped or about to strip your best bet is to replace the carb, if nothing else, to get a new tube, but might as well use the whole carb, get the right size jets, and start over, but you could remove the tube with the jet installed and solder the jet then redrill.
With the nt and nt speed carbs costing under $20 I would recommend a new carb, you could spring for a Dax RT carb for better performance for around $27 if you want trouble free performance. Personally I would stay away from the cns carb, there are other carbs out there that'll out perform it and they're easier to tune.