I know where to get one. But when can I get it?
you can find one:
-As a gift from amanita in castelia city(with its hidden ability) after defeating the Pokemon league. She is on the 3rd floor of the building on the same street as the gym.
-in castelia park after you defeat team plasma in castelia sewers. This video will help you find the park: http://www.youtube.com/watch?v=f243tM8PoIQ