On Amazon for 39.95
http://www.amazon.com/gp/aw/d/B00ZZD2OWO
On eBay it is about 7 dollars more expensive. You might want to hold off and see if someone is selling it.
You also have aftermarket options. Simple Google lg g4 battery cradle.
I bought one on Amazon that comes with a separate battery charger, and it's wonderful. I can't tell a difference in the battery life of the old battery compared to the new one, but hey, I have two batteries! Sadly, the price has nearly doubled since I originally bought it, but it may still be worth it to you.
LG G4 user here. Get the new battery. This One comes with a charging box. I can charge two of them on my Anker charger, and swap them as needed. I never have to plug my phone in anymore.