If you want a case to carry around an extra battery why not just get a large extended battery and be done? It would probably be about the same thickness and wouldn't require charging two separate batteries.
Does Samsung make a factory extended battery?
buy a battery from Samsung. It comes in a plastic case.
Off Topic, but is there a higher capacity battery that can go into the GS4, but not change the shape or thickness, so I can use my current case, and keep the factory samsung back cover?
So no one knows where to get a case for the battery?
If you get the official battery it comes with a case. Sorry if that doesn't help much.
Something like this? Seems thin enough for a pocket.
http://www.amazon.com/Samsung-Battery-Charger-2600mAh-Included/dp/B00BW0XZO0/ref=sr_1_1?ie=UTF8&qid=1377282914&sr=8-1&keywords=samsung+s4+battery
If you read the reviews, this particular one seems to be a knock-off. Some people, however, feel compelled to post in the comment section even though they did not purchase from this particular vendor on Amazon - ZHIJIANG TAO -
How about this?
http://www.amazon.com/Pocket-Purse-...UTF8&qid=1377284364&sr=8-58&keywords=pill+box
I don't know the exact size of the Samsung battery so I can't tell if this is large/small enough.