Go to Pacifidlog Town and go inside a house with a man looking at the wall. If he says he can see Mirage Island today, surf to it (near Routes 130 and 131). You will run into wild Wynaut. Farther into the island, you will find an egg that will hatch into Celebi or JIRACHI. Also you can use or trade from Colluseum Bonus Disc or use Action Replay.