Here is what I copied off of the Website for the 870 battery:
3S 6500mAh 11.1v 20c Lipo battery (Max size: 73mm X 80mm X 132mm)
You could use a battery with a little less mAh. Since you know the dimensions and that it is a 3S, check out this link for a battery:
https://hobbyking.com/en_us/batterie...ll-brands.html
They have tons of batteries with all of the dimensions listed.