I use those USB power packs, ones rated for at least 2.4A @ 5v. Like the kind you use to charge your phone when not near an outlet. They're readily available from amazon and the like.
Note that the capacity is spec'ed as the capacity of the BATTERY - a 12,000mAh pack means 12,000 mAh at ~3.7v - assuming 90% efficiency in the DC-DC converter, that means only 8,000mAh at the 5v output at best. And yes, this is blatantly dishonest marketing.