Downloadable Content in Pikmin games can be purchased from the Nintendo eShop in the Wii U Version. The Wii U eShop is closed after March 27, 2023 which made unpurchased DLC for it lost. I’m afraid if you haven’t got DLC for a Pikmin Wii U game, it is far too late! If you already managed to get all DLC for a Wii U game belonging to this series, then no worries. DLC in Pikmin games can only be available after you start playing and save the game for the first time before quitting.
To learn about the Downloadable Content in Pikmin 3, see Pikmin 3 DLC.