If you have talked to everyone then go to the northeast corner of 7th street and talk to a guy near a crate
EDIT: I'm not sure about this though, I don't remember clearly how I got to the sanctum
Smeargle has the move "Sketch" which copies the last move used in battle, so if the last move used was the one you were looking for then you can then use sketch and breed that smeargle.
Sometimes I think, is this all there is? Is life just, some horrific joke, without a punch line? That we're all just biding our time until the sweet, sweet release of death? -Grunkle Stan