Where to find Lapras in Sun, Moon, Ultra Sun & Ultra Moon

Surf: Poni Wilds (5%, Only in Sun & Moon), Ancient Poni Path (5%, Only in Ultra Sun & Ultra Moon), Poni Breaker Coast (5%, Only in Ultra Sun & Ultra Moon)
SOS Encounter: Poni Wilds (Only in Sun & Moon), Poni Breaker Coast (Only in Ultra Sun & Ultra Moon)
Gift: Poké Pelago

Lapras also appears in

How to get Lapras in Sun, Moon, Ultra Sun & Ultra Moon

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 Dive Ball.

Lapras in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)