ISI makes a "whipper" specifically for nitro coffee that comes with the nozzle, but they say you can't use it for whipped cream, and they don't sell the nozzle separately. Further, I believe they make their own N2 cartridges a different size so they aren't compatible.

It's true that N2 cartridges are much higher pressure (1500+ PSI) vs N2O (750 psi) but the standard cream whipper has no problem with N2. I think they're just trying to force people to buy additional hardware, so they can go pound sand. This siphon has a diffiuser nozzle, is cheaper than the ISI, and it uses standard N2 cartridges:

https://www.amazon.com/Impeccable-Culinary-Objects-ICO-Aluminum/dp/B0B5BX3NL4

I don't see anywhere you can buy a diffuser nozzle alone.