you get the master ball once you beat Cyrus in the galactic building
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action replay is best
cheating
you get unlimited master balls by using action replay and put on master balls x999
the same place as in diamond.
It can be got through the action replay code of Pokemon Modifier. But if you want Master balls to catch pokemon, add this code: 94000130 fcff0000 b21c4d28 00000000 b0000004 00000000 00000f4c 03e70001 d2000000 00000000 Press L + R to activate.This will give you 999 master balls.