For those of you looking for a spare battery for the Lumia 950 XL, I just bought two from this vendor. I'll let you know if there is any problem.
Replacement Battery BV T4D FOR Lumia 950 XL Cityman Lumia 940 XL RM 1118 3340mAh | eBay
Replacement Battery BV T4D FOR Lumia 950 XL Cityman Lumia 940 XL RM 1118 3340mAh | eBay