They do not double as a charger, or at least none that I am aware of do. See, you don't hook the battery into it at all. The battery is the "cigarette" looking portion of an e-cig and the "filter" part is the cartomizer and that is where the flavor / juice is stored and where the vapor is created. On a PT, you hook up only the cartomizer. The other really nice thing is that with the PT, the e-cig is very light. The batteries can seem a bit heavy, especially at first, but you get used to them quickly. The PT, however, is very light and I like that.
As far as where to get one, the vendor that you buy your KR808 will probably have them available. I purchased mine - and a starter kit, for that matter - from Vapor 4 Life (V4L) and couldn't be happier with their products or services. You have to figure out what is best for you, of course, but you'll find a great many V4L fans here. I'm one of them.
Also, I would recommend that even if you go with Automatic Batteries, consider getting a manual PT. I have tried both, and the consensus here seems to be that the manual is superior in both performance and longevity.
Something I didn't know until very recently - and something I wish someone had told me - is that if you're going to refill your own cartomizers with eliquid, you will probably have better luck and fewer hardware problems if you also go with manual batteries. As I understand it, the manual batteries are protected, and by that I mean they don't have the same problem with juice dripping down inside them that can happen with automatic batteries. If you are only going to use pre-filled cartos, then it probably won't matter which style battery you get. But, if you think you might want to refill your own, go with manual batteries. I have to order some myself today.
Hope this is helpful. Holler back with any other questions.
Sherrick