iPhone 6 Battery Case, MFi is fantastic as a short road trip battery. It's bigger than I want to carry when flying but is perfect for charging. Right now Alpatronix has their iPhone 6 Battery Case, [MFi Certified] Alpatronix BX140 Ultra-Slim Protective Extended iPhone 6 Charging Case with Removable / Rechargeable Power Cover [Fits all versions of the Apple iPhone 6 / 3100mAh Battery Pack / Full iOS 8 Compatible Support / No Signal Reduction / Apple MFi Certified / 4.7 Inch-Model Only] - (Silver - Purple & Clear Color Bumpers) for cheap price. You'll just need to visit the site.
Alpatronix BX140 Extended Protective Battery Case for iPhone 6 (4.7 inches) Overview: The BX140 is designed by Alpatronix to improve the performance and battery life of your iPhone 6. Built for the avid iPhone user and extensive use, the BX140 is excellent for traveling, work and on-the-go protection.