After you obtain your eighth badge, and is well on your way to the Pokemon League, she will stop by to hand it to you.
(I apologize, I forgot the name of the city with the neon lights and floors everywhere, I'm still getting used to the new names.)
You get it after you beat the Oplucids Gym leader and when you get to the Gate leading to the Victory Road/Pokemon League, Professer Juniper will Come by and then Give it to you
