Where do you get a macho brace in FireRed?

You can't get a Macho Brace in Pokémon FireRed. The Macho Brace was introduced in Generation IV (Diamond, Pearl, and Platinum).

It is possible you are thinking of a different item. Do you perhaps mean the Macho Mask? The Macho Mask is a held item that can be found in FireRed, but its effect is to increase the power of Normal-type moves, not to boost EV training like the Macho Brace.