A while back, Nintendo announced that some games would be able to take advantage of using two Wii U GamePads for gameplay, but at some graphical costs (which is understandable, as it takes quite a bit of power to maintain just one GamePad with all of its bells and whistles).
Unfortunately, as of right now, GamePads will not be available separately, as the guys up at Nintendo has stated that it would confuse consumers since none of the launch titles would be utilizing more than one GamePad. So, if you were to want to nab another one of these pretty cool controllers, you’ll have to get one straight from Nintendo of America. Of course, this could all change once games begin to utilize the new dual-GamePad feature, but for now, you won’t be able to find one in stores come Nov. 18.
Also, the Wii U Pro Controller, which closely resembles an XBOX 360 controller, will be available from launch at the price of $49.99 USD, and the Wii U will indeed be able to support 4 of these types of controllers.