How to Get the Master Ball
Successfully complete the game. After the credits, you will be sent back home and into your room. Leave your room, and a woman will enter your home and talk to you. When she is done, she will give you a Master Ball as a reward for becoming champion. There is only one in the game — so make sure to use it wisely. It is recommended to use it on the legendary Pokemon so it does not take a long time to catch.