I've purchased them from eBay.
Do a search for CGA320 for CO2 or CGA580 for N2.
You have to measure your cylinder to see what size valve you need. They come in a few different thread sizes. I recently found one that threaded into a 20 oz paintball tank. (rare)
Also...just a recomendation. Check with your Gas Supplier before getting the N2 valve. They may put beer mix in a tank equiped with a CO2 valve as well. Unless you feed a Gas Blender...the CO2 tank with Beer mix would work just as well...and you'd be able to use standard co2 regulators to feed your tanks.