After beating Crasher Wake in Pastoria City, you shall see a Team Galactic Member somewere near the Gym. Talk to him then he'll run away,follow him until you he faces you in a battle. Then Cynthia will appear and give you the SuperPotion.
If you are looking for any help visit this forum i made on Psypokes.com were i will be asking questions about the game and problems i am having.
Link:http://www.psypokes.com/forums/viewtopic.php?t=14239 or look for the title "I can't..."