1. Challenge the Pokemon League and fight through to the Champion. If you defeat the Champion, you will receive a Master Ball as a prize.
2. Exchange 200 Battle Points for a Master Ball. This can be done at the Battle Point Exchange located in the Pokémon Centers in Castelia City, Driftveil City, and Lentimas Town.
3. Find Master Balls as hidden items. Master Balls can sometimes be found hidden in the overworld, such as in a tree or in a rock. You can also find Master Balls in some Pokémon nests, which are only accessible by using the Hidden Machines Rock Smash or Cut.
4. Receive Master Balls as gifts. Sometimes, you may receive a Master Ball as a gift from a non-player character (NPC). For example, you may receive a Master Ball from the mayor of Nimbasa City after completing certain quests.