Amazon has iXCC 10000mAH Battery Pack for $24.99 with free Prime shipping ($45 off). Coupon code IXC20OFF drops an additional 20% off, pushing the price to $19.99 shipped. This high capacity external battery power bank has both DC 5V/1A and DC 5V/2.1A USB outputs for both phones and high capacity tablets and a 1 year warranty.
Make sure to use Prime (free trial, 6 months for college students) to get it by Christmas. With the money you save you can stock up on Lightning and/or microUSB cables and still have some money left over for another $30 google Chromecast.