Question

I need planks for my secret base.

Answer 1

Bulbapedias entry on Lilycove Department Store

Randomly, after beating the Elite Four, Lilycove Department Store will advertise their rooftop sale by using the Pokémon News station. The times where a sale goes on is quite rare. However, the items sold in the sale cannot be received by any other method, other than from the trader.
Hope this helps ^^