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

Super Rod: Paniola Town (99%), Seaward Cave (99%), Route 13 (79%) + 20 more
SOS Encounter: Akala Outskirts, Route 13, Route 14 + 17 more
Surf: Route 8 (60%), Route 14 (50%), Route 15 (50%) + 17 more
+ 1 more way

Magikarp also appears in

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

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, Poké Ball or Sport Ball.

Evolution: Magikarp → Gyarados (Level 20).

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