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

SOS Encounter: Thrifty Megamart
Walk: Thrifty Megamart (40%)

Haunter also appears in

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

Its base catch rate is 90 out of 255 (higher is easier); the catch rate calculator works out your odds with each ball. For the look, the best colour match is Dream Ball, Dusk Ball or Master Ball.

Evolution: Gastly → Haunter (Level 25) → Gengar (Trade).

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