Previously listed unofficial Nyko £30 PowerPak reached 233°, so I guess many will be interested in the new OFFICIAL £22.84 Nintendo GamePad battery. There are some problems with Nyko's product, official one should be much better. So far its available only in Japan and its unknown when or whether it will be available in US or EU. It will not be much cheaper anyway.
Original battery in GamePad: 1500mAh, 3-5 hours.
Official Nintendo GamePad Battery Pack: 2550mAh, 5-8 hours.
Delivery is essentially free ($0.50 economy air box) and should be customs free because of low value.
So the total you pay is £22.84 ($34.9+$0.5=$35.4).