You can get Master Ball from Prof Elm after you beat all eight gym leader. He'll call you.
What he said
there are like two ways to get the masterball,
one: Prof. Elm calling you.
two: winning the lucky number show i think a few times
do u have to beat the elite 4 as well??
cuz Prof. elm hasnt called me. Hes fulla sh*t.
And I beat all 8 gym leaders.
I think I only won the lucky number show like once..