Nowadays it's hard to find an official Nintendo Gamecube controller adapter without having to spend much more than it's actually worth. Fortunately, Play-Asia has a four port Gamecube adapter in stock and costs only $19.99 (not including shipping).

The adapter supports Super Smash Bros. Wii U, has no delay and has vibration feedback. There is also a switch on the side that let's you change from Wii U mode to make it compatible on the PC.