As it turns out, we don't carry the back battery cover for the iPad Air 2 at this time.
However, you may want to try PowerBookMedic, Amazon or eBay. If you do manage to get a hold of one, our guides may be able to assist, but if we don't even have a guide, I would encourage you to make a guide for other people to use!
If you're still unable to locate the item, I recommend posting to our Answers forum as one of our community members may know where to source it.
Best of luck to you, and happy fixing!