Gift: Iki Town (Only in Sun & Moon), Route 1 (Only in Ultra Sun & Ultra Moon)
You won't find Popplio in the wild in this game — you get it as a gift.
Looking for Brionne? It isn't found in the wild in this game — evolve Popplio (Level 17).
For the look, the best colour match is Dive Ball.
Evolution: Popplio → Brionne (Level 17) → Primarina (Level 34).
Popplio in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)