I have seen one on the site listed above, and an one on bestbuy.ca. both with larger case backings which is expecte and I'm fine with. I haven't seen any Samsung brand extended batteries however. I'm out and about alot and would like my phone to hold a charge for a whole day. I've a couple friends with the S4 mini and all of us do quite a bit of talking. They usually have to charge their phone half way through the day. I would like to avoid that. The other thought would be to see if someone has made a solar panel backing for the phone but I don't see that being very efficient.
I know this is an old post, but maybe someone will stil find this information usefull. The absolute best battery for the S4 Mini is the Zerolemon 5100mAh one. You can find more informations about it in this video that I made: https://www.youtube.com/watch?v=hN-UKjj0urM