You can get it from a girl in the Slateport market who's standing next to the person who sells vitamins. I think the easiest way to get it is to buy lots of vitamins for every stat and feed 52 of them to the Pokemon you want with the effort ribbon. If that isn't the easiest way for you, you could try giving the Pokemon a macho brace (obtained by defeating the Winstrate family) and have it defeat random wild Pokemon in Victory Road.
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