I bought a battery off of Amazon, it was crap, it had half the life of my old battery. I ended up putting my original battery back in. None of the sellers on Amazon have consistently good reviews, all of them have significant numbers of one star reviews with people reporting the same experience that I had. I tried to buy one directly from LG but they won't sell you a battery, they insist that you send your phone to them.
I've decided to wait until the 2015 phones come out and then replace the N5. Hopefully the phone lasts until then.