It appears that Nintendo has completely sold out of the GameCube Adapters on its site. However, GameStop is now selling a third party version by Nyko. As of this writing it is in stock for $19.99. Check it out here.
We went through this same shortage on the Wii U when Super Smash Bros. launched. It was hard to find the GameCube controller adapter back then and it is now. Nintendo’s online site has it in stock (as of this writing) for $19.99. This allows gamers to hook up to four GameCube controllers to the Switch and play Super Smash Bros. Ultimate with the familiar controller. Get yours now before they run out again.