Only in Ultra Sun & Ultra Moon
NPC Trade: Route 8
You won't find Arbok in the wild in this game — you get it in an in-game trade or by evolving Ekans (Level 22).
For the look, the best colour match is Heal Ball, Master Ball or Dream Ball.
Evolution: Ekans → Arbok (Level 22).
Arbok in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)