you snag it from precious little miror b. after you snag every other pokemon in the game (you dont have to purify them all). he'll be on top of the lighthouse in gateon port dancing with his ludicolos.
HE'S COOL! YAY!
ahem, sorry. just wear it down with your weakest attack, then throw ultraballs at it. if you can't catch it with those, try timerballs.
then miror b. runs away... *sobs*