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!
Source
Hope This Helped :D