Our USB Power Packs use two 18650 Li-ion rechargeable batteries rated at 2600 mAh each.
If you need to replace your batteries, they can be purchased from any number of suppliers online. We do not often buy them in small quantities, so have no recommendation for a specific source. You can trying searching Amazon or elsewhere for “18650” - be sure to search for protected li-ion batteries.
This is also written on the battery in the power pack - which can be accessed by sliding apart the 2 pieces of the battery pack. Yes, they do come apart ;)
Here's a handy link to the power packs - they are very useful especially when you are at festivals or traveling - http://www.flowtoys.com/usb-power-pack/
Please feel free to contact us if you have any questions!