CanadaAmmo sometimes has it. Unfortunately only Fiochi loads it, and they only do limited runs of their exotic stuff (Nagant, 9mmStyer, .455 Webley etc), so there are long droughts between availability.
Tradex has some components if you reload.
I suggest a visit to: thenagantman.com. He is a Nagant revolver fanatic and has a bunch of youtube video with a list of compatible calibres and a re-loading recipe. The .32-20 is apparently the right case, and what the Lee die set was designed to use. I haven't gone this route myself yet since I still have four boxes of factory ammo left, but I expect I will have to do so soon.