Since most battery packs are 1700mAh or 2200mAh, you'll need to find something beefier. At the moment, the ZaggSparq is the most powerful on the market, with a 6000mAh battery. Costs $99.

Alternatively, you could get a pack like the Tekkeon MyPower: http://tekkeon.com/products-mypowerall.html

At 5V, this thing will deliver 10000mAh, plus it can power a laptop as well. With that kind of capacity, you are looking at about 7 full iPhone charges.

EDIT: Just noticed the iMaxPower packs previously mentioned; they go up to 8000mAh: http://www.imaxpower.com/