You go to Vielstone city and go into Team galactic building and find Cyrus(Team galactic boss) then battle him. After the battle he should give you a master ball.
master ball
no
you can not get a master ball in Pokemon platinum only the one professor Rowan gives you
with a master ball or timer ball
there is no more masterballs in Pokemon platinum but not if you use cheats
Master Ball.
with a master ball
After defeating Cyrus in Veilstone, he should give a Master Ball to you.
Master ball
Ultra Ball
In Pokémon Platinum, Cyrus, the leader of Team Galactic will give you a Master Ball once he surrenders his plans in his Veilstone Headquarters.
not possible