I don't think it is possible to get a replacement battery. I contacted Pocketbook USA, and they gave me a price and I sent in my Paypal money (about $22). I have not heard from them since - and it has been a couple of months. No communication whatsoever, so not sure if they are still trading. They certainly have not responded to any of my emails asking what has happened to my battery. Even Paypal cannot help me now as I left it longer than the 45 days allowed.
Bit disappointed in Pocketbook now - might look at another company for my next ebook reader.