How do you get a master ball in Pokemon Black and White?

Answer 1

Once you beat the eighth gym, go outside of the gym then,then after that go to castelia city and trade with 50 people around the world then talk to a man on top of the stairs from the Pokemon center and he will give you a master ball hope this helps!!! :)

Or you get one from Professor Jupiter after getting the 8th bradge.

Answer 2

After earning all 8 Unova Badges, talk Professor Juniper in Opelucid City and she will give you a Masterball. You can get a second Masterball by talking to a man inside the Pokemon Center at Castelia City after you have obtained a traded Pokemon not originally from your game that has been traded over 50 times.