Professor Elm gives you a Master ball after you defeat the Blackthorn City Gym.
Alternatively, you could give the Master Ball to a Pokemon to hold on a game that has already received the ball, then trade said Pokemon, with Master Ball, to the newer game.
Winning extra Master balls is possible by matching all five numbers with the ID Lottery at the Radio Tower in Goldenrod City. The chances of winning them this way is very slim, however.
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You find it in the team aqua hideout.
One masterball is in Pokemon emerald. To find out more go to my site called www.miquel.ning.com
Simply get a Gameshark or Action Replay. Then go to their website, find Pokèmon Pearl, and look for the Masterball cheat provided. (The Masterball cheat should be put on the device already, however).
Other than the masterball try using a lot of ultra balls.
if u use any masterball cheat u get bad egg in Pokemon PC