Not necessarily what you were after but those cases tend to be quite bulky and expensive, while cheap ones are available on eBay, they will be crap, guaranteed! A better option if your worried about the battery life would be something like this:
Anker® 2 x 2600mAh Li-ion Batteries for Samsung Galaxy S4, GT-I9505 with S Beam + Anker® Travel Charger [18-Month Warranty]:Amazon:Computers & Accessories
IKnow they have the same thing available in America, reliable batteries and fit as a stock replacement so not adding unnecessary bulk that those cases add and it would triple battery life! Or double if you just carry one spare!