There are only 2 places where you can get a Master Ball in Platium.
>Galactic Veilstone Building: Defeat Cyrus
Ok, so you already got this one.
>Jubilife City: Match all 5 numbers in the Pokémon Lottery Corner
This one is harder, and it depends on luck. But be patient, and one day you might match all of them!
Hope This Helped :D