Another third-party company has decided to release a Gamecube Controller Adapter for the Wii U. Gtron’s latest product was recently made available on Amazon. You can purchase it now with this page. Check out the photos of the latest third-party gamepad converter below this paragraph. 
Like the other adapters, the Gtron Gamecube Controller Adapter is connected to the Wii U using two USB cables. You will be able to use four Gamecube controllers per converter. Since Super Smash Bros. for Wii U can support a maximum of eight players, you can also add another adapter as the Wii U has two USB ports located on both the front and back sides of the consoles (four total).
Nintendo has been having trouble with trying to meet the huge demand of the official Gamecube Adapter for Wii U. This issue is forcing many gamers to look for third-party alternatives that are cheaper than having to pay a huge markup from scalpers and resellers to get the first-party version. The most popular substitute is adapter from Mayflash, which you can check out on Amazon with this link.
The only game that supports the Gamecube Adapters so far is the above-mentioned Super Smash Bros. for Wii U. Of course, Nintendo could announce more supported software in the future.