First things first: here’s the listing on Amazon. You are welcome.
This controller adapter lets you connect your GameCube controller to the Wii Remote, allowing you to use your GameCube controller for games compatible with the Wii Classic Controller and the Wii Classic Controller Pro. This means that we can play games like Super Smash Bros. Brawl on Wii U with GameCube controllers. This also means that we can play the next Super Smash Bros. with GameCube controllers.
The adapter is listed at $18.99, and is customized with Turbo for the A, B, X, Y, L, R, and Z buttons. I am not sure what that means, but I do know that I can play Brawl with GameCube controllers.
In case you forget, this means that we can play some Smash with the best controller ever made.
Source: Nintendo World Report