You can find Miror B early in the game. The first time you encounter him, he will ask you to join his group. If you say yes, he will think that you are lying. Say no, and he will get mad. Either way, you will have to battle him. He has a Shadow Nosepass. The second time you encounter him, he will be on an island. When you talk to him, he will battle you again. This time he has a Shadow Dragonite, and also the Shadow Nosepass if you did not catch it earlier.