How to get a Master Ball in pokemon white:

First of all, head to Castellia City. Next, walk into the pokemon center. If you look in the top left corner, you will see a man in a black suit. Talk to him an he will ask you how many trades you have done. If you have done 50 or more trades, he will give you a Master Ball. You can also get Master Balls by trading or earning them from Professor Juniper. Well, I hope this advice helps!!!