I bought an 3800mAh extended battery from aliexpress for just under 10$.
I must say that I'm amazed by the performance.
I have been using the phone heavily the past 3 days.
A little GPS navigation, talk time, browsing, etc.
Here's the link to the product and a few screenshots!
I highly recommend this product for heavy users.
The phone becomes a bit bulky but that's the price you have to pay.