How do I get a Master Ball in Black/White?

Question

I read that you talk to the guy in the Pokemon Center and trade 5 Pokemon from the GTS and I did that but...Instead of giving me a Master Ball he gave me a...WIDE LENS...Why?

Answer 1

1. From Professor Juniper in Opelucid City after obtaining all 8 Gym Badges
2. From a man in the Castelia City Pokemon Center for having 50 Pokemon with unique OTs

There are no repeatable methods of obtaining a Master Ball in these games.

Master Bull (Bulbapedia)

Answer 2

You get a master ball from professor juniper in castelia city and/or mistralton city.
The person who gave you a wide lense will give you different items depending on how many times you trade with gts or gts negotiations.