You don't, as there's only one Master Ball in the game. You can hack to get more, but otherwise, there's no way to do so normally.
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Professor Elm calls you as soon as you leave the Dragon Den in Blackthorn City. Fly to New Bark Town, and he'll give you a Master Ball. This is the only Master Ball in the game. The only way to get another one is to hack or trade to another game having that game's Pokemon holding the Master Ball. EDIT: You can also get a Master Ball by winning the lottery on the Lucky Numbers Show.
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