I am not sure if this is available at all the Staples. I found a Tzumi Pocket Juice PJ2200 powerbank in the clearance section. They may be small and not overly powerful but for $1.00 each they seemed like a pretty good buy. They are a 2200mAh Lithium Ion battery rated at Input/Output of 5V/1A. They also include a built in, small LED flashlight. The paperwork says it can charge "most smart phones 1-2 times" It is small enough to pocket or fit in a laptop or EDC bag.
Again, not sure if they are available everywhere, but seems like a pretty good buy to me. Hard to beat the price.