This article shows the In-Game location of Feebas, the basic form of Milotic for Pokémon Sun and Moon.
Feebas spawns at Brooklet Hill (Fishing Spot) in Akala Island with a very low spawn rate.
A recommended ball to capture it is a Net Ball since it’s effective against Water-Type Pokémon.
How to get Milotic
Feebas will evolve into Milotic when traded while it’s holding a Prism Scale.
You can obtain a Prism Scale from Exeggutor Island found in Poni Island.
Advantages in using Milotic
Milotic has high base Sp. Defense, and by allocating EVs to Defense and HP, it can be used as a defensive Pokemon.
In addition, Base Sp.Atk Stat is also commendable which makes it effective as an offensive Water-Type during the Main Story Battles.
Though both of Milotic’s ability is very useful, it is recommended to raise one with Competitive instead since it patches up its already above-average Sp.Atk Stat when one of its stats is lowered.