As one should expect when owning a device for almost two years now, my iPhone 4S's battery is beginning to show its age. I am looking for a good replacement battery that is affordable, unlike Apple's $80 charge for replacing it. Does anybody have suggestions for a good replacement part that has served them well? There are dozens of sellers on eBay and Amazon who sell 3rd party batteries, but reviews are always mixed. Or, alternatively, does anybody know of an online store that sells authentic (for sure) Apple batteries for the 4S? Thanks!