How do you get plank in Pokemon ruby?

After you defeat the Elite Four and the Pokemon League Champion, you must watch a local TV, and if you watch enough, an event should occur involving the Lilycove Department Store. Head over there A.S.A.P., this is where you get rare secret base items, including the plank. Note that this event is completely random, you must get extremely lucky to encounter it. The result is, you have rare secret base items to show off and brag to your friends!