SOS Encounter: Exeggutor Island (12%), Haina Desert (12%), Lush Jungle (12%) + 4 more
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 Premier Ball or Dive Ball.
Castform in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)