SOS Encounter: Blush Mountain (15%, Only in Ultra Sun & Ultra Moon), Route 1, Route 4 + 2 more
Walk: Route 5 (20%), Route 1 (15%), Route 4 (10%) + 1 more
Its base catch rate is 255 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 Repeat Ball or Fast Ball.
Evolution: Grubbin → Charjabug (Level 20) → Vikavolt.
Grubbin in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)