Is it possible to buy a genuine iPhone battery online? I searched for an iPhone 3GS replacement battery on Amazon and while many items claim to be original/genuine, a lot of reviewers still complain that they are not/not as described/do not work well. Is going through Apple the only way to get a genuine battery or is it possible to order one online?
I've replaced several batteries in older iPhones including the iPhone 3.. I have not tried in the iPhone 4 models.

In the original and the iPhone 3 / 3GS, it's really quite simple. The batteries were fine (from Amazon). They come with the tool to separate the back panel, look at iFixit.com for instructions, but just take your time and be patient..

HTH