When you beat the last gym you go towards the route north of Oplucid city the lady at the desk tells you there's a storm on some route. explore. when you get to a house a lady will come out and give you some soup. when shes done talking head east a pokemon will float down in front of you. when he leaves dont forget to save. it will stop walk around in the grass until it starts to rain again. when it does explore the grass some more. your sure to find him soon enough. (only for white version)