I got mine from e-bay for 12.95. Its a 3600ma and I use the app battery calibration. The stock battery was just not working as well as I thought it would in fact the battery for the Admire was giving better life than the Motions stock one.
And yeah it makes your phone a little bulky but you get used to it in a day. I went with e-bay because I had money on a paypal account from winning a fantasy football league, I'm sure I could have gotten a better deal if I had took my time to look around but I wanted to get it right away so I spent the money. I'm waiting to swee how well the Calibration app works on the Motion, I had it on my Admire and it worked great.