you can get master balls at the end of the game when u beat the elite four and you should use the master ball on reshrim....... y did u ask this it is pointless and it tells that u have never beat the game ^_^
You can get 2 Master Balls in both versions.
You don't.
In skyrim
Ar code
you trade 100 times
you can't
hack the game
you can trade a Pokemon from black or white holding a master ball.
the professor will give it to you after you have beaten the 8th gym.
action replay codes
u can only get one master ball in Pokemon Black to my knowledge, unless u trade with someone whose Pokemon is holding a master ball, or if u hack it, which there is no hack to.
in the pokemon league after beating the elite 4 talk to each 5 times them YOU HAVE TO BEAT THE CHAMPION WITHOUT LOOSING 1 POKEMON AND THEN GO TO THE BLACK CITY mart and you can buy master balls