Where do you look for the move surf in Pokemon diamond?

Cynthia's grandmother gives you it after the bomb scare, which is after the Psyducks-ness. You need the SecretPotion to cure the Psyducks. You get that from Cynthia after you battle the Galactic Member you're supposed to stalk from Pastoria (The location you get the the SecretPotion is at the Valor Lakefront).