SOS Encounter: Thrifty Megamart (15%)
Its base catch rate is 45 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).
Gengar in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)