Fiio said to buy it here and put the order quantity at 15.
http://www.aliexpress.com/store/product/Extra-shipping-cost/1473108_32233147015.html
Thanks for the link, but I'm a little confused. The page is just "extra shipping", nothing about batteries. Did see this link though: http://www.aliexpress.com/store/product/FiiO-X3-Spare-parts/1473108_32234108503.html.
This is he email I got from FiiO
Thank you for your mail and support to FiiO!
Yes, you can buy the battery from us. You can pay it in this website. http://www.aliexpress.com/store/product/Extra-shipping-cost/1473108_32233147015.html
Just change the quantity to 15 and tell us your name, address, postal code and tel. We will choose post office to send it to you once we get your pay. I send you the instruction.
If you have any other question, please feel free to contact us or post a message on FiiO forum (http://www.fiio.me/forum.php) directly! We will reply you ASAP.
Have a nice day!
Best Regards,
FiiO Customer Service