Answer 1

A person in the Goldenrod Department Store will ask for a Drowzee in exchange for a Machop. Do the trade. The Machop holds the Macho Brace.

Comments

What level is it?

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Whenever you do in-game trades, the level of the pokemon you get is always the same as the one you traded. I actually just did this yesterday, I gave him a level 14 Drowzee and got a level 14 Machop back.

Answer 2

You can also get it in the Battle Fronteir in exchange for Battle Points that you can get in the battles there.

EDIT: Yay, this is my first answer!