Walmart stores have the 10,000mAh Tzumi power pack for $5 on clearance (retail $29.99). 3x Lithium cells inside, shock and water resistant with the brightest LED flashlight i have seen on a power pack.
Brief PSA about lithium batteries:
This pack almost guaranteed to be less than 10,000mAh, I havent opened it up yet, but the cells for such a pack are simply more expensive than this packs pricepoint. Regardless, most packs overstate their capacity and for $5 this is a great deal
Don't EVER charge a lithium battery that is below freezing. This is a great way to kill your battery.
Also, don't throw this in a drawer discharged and pull it out months later expecting it to still work. When discharged too low, lithium battery packs will not allow recharging (for safety) although self-discharge is slow, if the pack is drained it might not take long for the pack to go below the allowable voltage for recharging.