Nintendo have begun selling the Wii U GamePad as a standalone product via their online shop in Japan. Of course, you still need a Wii U and you only can connect only one GamePad at a time (though Nintendo have hinted at future dual GamePad support for quite some time).
The product only works with Japanese Wii U systems and will not be sold in regular shops. It's available in white and black. The price is 12.800 Yen which is about £70. It will ship with battery pack (WUP-012) and Stylus (WUP-015). But the AC Adapter (WUP-011) is not included. The main reason for selling the GamePad, is for players to be able to replace a damaged Wii U GamePad without having to buy the entire system.