The Wii U ships with a single GamePad because that’s all it supports. If other players want to join in on the action they have to use a Wii Remote. You can’t buy a new GamePad, Nintendo does not offer them. If yours breaks then you only have two options: contact Nintendo directly for a replacement or go out and buy a new (or used) Wii U.
It’s been that way since the Wii U launched in November 2012, but Nintendo has finally decided that selling the GamePad separately is a good idea. Unfortunately, they are only going to do this in Japan from November 24, and the cost is a rather steep $112.
Once on store shelves in Japan, importers will surely pick up some stock and offer it (with a healthy premium) to gamers in the US and across Europe. Oh, but wait, they can’t as Nintendo has region locked the controllers! It is ludicrous that Nintendo waited 3 years to do this, still won’t offer them outside of its home territory, and never found a way to get the Wii U to support more than one at a time.
Hopefully, with details of the NX dropping next year Nintendo will have learned from its mistakes and offers a console that supports multiple controllers once again rather than relying on previous generation hardware.