At the bottom of the full Alolan dex, you see the Ultra Beasts. They have slots in the Pokedex before you catch them (I think, but I'm 90% sure they have slots, I remember). Unlike Marshadow, Zeraora and I think Magearna too, their slots aren't there until you actually obtain them so they're not needed. It shouldn't be too hard for you to get all the Ultra Beasts, except for version exclusive Ultra Beasts... Good luck!
I hope this helps! I'd like to know if I made any... Mistakes.