The only way I know of is before you challenge the final four the professor gives you one.
hack the game
action replay codes
You can get 2 Master Balls in both versions.
You don't.
you can get one when you defeat iris or do 50 trades an talk to some black suited guy in castelias pokemon center
In skyrim
you can't
you trade 100 times
Ar code
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.
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.