Stouts are carbed up with c02, just like other beers. Some are even (gasp!) bottled.
Seriously, I don't have a nitro tap and never will. My stouts are great, and I don't need beer gas or anything except my regular set up. There are some things people can do to sort of imitate the creamy nitro feel if they want- use flaked barley in the recipe, carb it a bit lower, etc- but it's very good even without a nitro tap.