The PixelWhips Rev2-4 use a 18650 Li-ion battery, 2400 mAh that comes with a charger.  

We no longer offer batteries or chargers and separate accessories due to stock issues since early 2020.

If you just need to buy an extra battery and/or charger, you could simply order one from Amazon, or the Battery Store. You’ll need an "18650 Lithium-Ion, PROTECTED CIRCUIT" :