Question

Where can I find 2 Master balls in black?

Answer 1

1. Meet Professor Juniper in town after earning all eight Unova Badges.

1. Talk to the man in the Castelia City Pokemon Center after the player has outsider Pokémon with at least 50 different original Trainers.

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Just to elaborate on Question 2:

When you have 50+ Pokemon in your games from other trainers, talk to the guy on the second floor of Castelia City's Pokemon Center on the left-hand side. He is wearing a black suit with shades.

So just get 50 different Pokemon from 50 different people through trade. GTS, Infrared, Wi-Fi, anything. They can all be Patrats or whatever, just make sure they have different trainers. GTS can be a big help.
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(Note: you only get one from each person.)
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Source: Bulbapedia/ Masteball Locations

Comments

Devil guy where is the man you talk to and whats with the 50 trainers and outsider pokemon,after the player? sorry but you are kind of confusing.

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He means you talk to the guy after you have traded with 50 different people.