SOS Encounter: Route 17 (15%, Only in Sun & Moon & Ultra Sun), Route 10, Route 11 + 2 more
Walk: Route 10 (20%), Route 11 (20%), Route 17 (20%) + 2 more
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 Repeat Ball or Level Ball.
Evolution: Spinarak → Ariados (Level 22).
Ariados in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)