Where to find Shellos in Sun, Moon, Ultra Sun & Ultra Moon

Only in Ultra Sun & Ultra Moon
NPC Trade: Seafolk Village

Shellos also appears in

How to get Shellos in Sun, Moon, Ultra Sun & Ultra Moon

You won't find Shellos in the wild in this game — you get it in an in-game trade.

Evolution: Shellos → Gastrodon (Level 30).

Shellos in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)