The Master Ball is a very unique type of Poke Ball, catching a Pokemon without fail should it make contact with a Pokemon when thrown in a Wild Pokemon Encounter. Because it has the distinct ability to do this, unlike every other type of Poke Ball, Master Balls are extremely rare to obtain within any Mainstream Pokemon Game. Even in this game, a trainer can only obtain one single Master Ball during the Main Storyline.
In order to obtain the one and only Master Ball during the Main Storyline, you must defeat Team Rocket in the Silph Co Building found in Saffron City. Once Team Rocket has been removed from the Silph Co Building and defeated once and for all, the Silph Co President will reward you for your efforts by giving you the Master Ball.
Not sure when you should use your Master Ball? Check out our When To Use The Master Ball Page!
In addition to the Master Ball you will obtain from the Silph Co President, you can also find Master Balls in the Sparkling Spots found in Cerulean Cave. These Sparkling Spots randomly hold rare Items, so it isn't a guarantee that you will obtain a Master Ball when clicking on these areas. But since these spots Respawn every time you leave and enter the second floor of Cerulean Cave, players will have the chance at obtaining extra Master Balls each time they do!
Want to obtain loads of extra Master Balls? Check out our How To Farm Master Balls Page!