so are you looking for beergas, or pure nitrogen? pure nitrogen should ONLY come in a cga-580 tank, which is the same valve used for argon, i believe. beergas can come in either CGA-320 (co2 valve), or CGA-580, it depends on where you get it filled.
i've seen online places that sell nitrogen tanks which are just co2 tanks outfitted with a nitrogen valve. they are usually rated at odd tank sizes like 33 CF. native nitrogen/beergas tanks are usually steel, and rated in increments of 20CF.
you can purchase valve nuts/couplings on mcmaster. the regulator body has just a 1/4'' fitting, could be left hand or right hand threads, if you want to do this to a co2 regulator, make sure they are right hand threads. also, some regulators are rated to 1800 psi, others are rated to 3000. 3000 would be better for nitrogen. most tanks that you can purchase will only be filled to 1800 psi, i believe, though i think there are 3000 psi tanks.
before you purchase tanks online, make sure there are places local that can fill whatever you decide to purchase.