The Lenmar Maven BC6 3000mAh battery pack can charge the iPhone 6 and the iPhone 6S and is currently down to $10 on eBay from trusted seller quickshipelectronics. They have overwhelmingly positive reviews with most of the biggest complains surrounding the packaging and not the products they ship. The deal comes with free shipping and quickshipelectronics offers a one-year warranty as well.
This is a much better price than anywhere else right now, with Amazon being the next best seller with the same case in Blue for $19.
If you own an iPhone 5/5S/5se, you can get this Mophie Juice 2100mAh battery pack for only $25. That’s a match for its lowest price ever and a drop from around $36 normally.