Zerolemon batteries are good, but if you can pop the back cover off your phone and replace the battery, carrying a few spares with you is even better. Same power, but you can recharge 10 spares at the same time (in 10 chargers) rather than waiting many hours for a 20,000mAh battery to charge. (It would take a 20 Amp charger to charge it from flat [which you never do unless you want to make Zerolemon rich, it's just used as a comparative measure - a 1C charge from 0 takes longer than an hour] in 70-90 minutes.)
Oh, and Amazon anywhere sells them.