Amazon currently has the Anker PowerCore 13000 for $23.90:
This is the lowest price ever per CCC. It has pretty good reviews and an A-rating from fakespot.
- The Anker Advantage: Join the 10 million+ powered by our leading technology.
- Upgraded Capacity: The successor to PowerCore 10400—identical compact size but packed with even more power. Charges an iPhone 6s almost 5 times, a Galaxy S6 3 times or an iPad Air 2 once.
- Light and Compact: A super-high 13000 mAh capacity with two high-speed-charging USB ports is all contained in a portable charger smaller than a wallet.
- High-Speed Charging Technology: PowerIQ and VoltageBoost combine to deliver a max speed charge to any device (up to 3A - Qualcomm Quick Charge not supported). Input: 5V / 2A.
- What You Get: Anker PowerCore 13000 Portable Charger, Micro USB cable, travel pouch, welcome guide, our worry-free 18-month warranty and friendly customer service.