You can get that battery right here:
[product|IF123-054]
It's Model #A1582
Amazon has it here: https://www.amazon.com/s/?ie=UTF8&keywords=a1582+battery&tag=googhydr-20&index=aps&hvadid=153718567838&hvpos=1t1&hvnetw=g&hvrand=10306507597602778468&hvpone=&hvptwo=&hvqmt=b&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1026578&hvtargid=kwd-185752433330&ref=pd_sl_3ksfle5ct6_b
eBay has it here: http://www.ebay.com/itm/New-Genuine-Battery-A1582-For-Apple-Macbook-Pro-13-039-039-Retina-A1502-2015-Best-Type-/222002264751
AliExpress has it here: https://www.aliexpress.com/store/product/Laptop-Battery-a1582-Battery-For-Macbook-Retina-A1502-Battery-Later-2015-Year/1287142_32540361404.html
Alibaba has it here: https://www.alibaba.com/showroom/a1582-battery-for-apple.html
dvwarehouse has it here: http://www.dvwarehouse.com/A1582-Laptop-Battery-for-MacBook-Pro-13-Retina-Early-2015-Apple-Original---NEW-p-42211.html