Where is Leftovers in Pokemon Sapphire?

It's on the SS Tidal. After you beat the Elite Four for the first time, your dad (Norman) will give you the ticket to get on it. Go to the Slateport or Lilycove Harbour and take the ferry to the city you didn't choose ( from Slateport sail to Lilycove and vice-versa). Go down the stairs in the top-right hand corner and check all the bins. Leftovers will be in one of the bins.