I picked up a five pack of the "Power Bank" cases Stephen links above, it was about $2.50US. Then grabbed a four pack of the LiPo cells for around $12.00US.
They work pretty well, but I would be surprised if the rated 2200mAh of the cells is right, they don't seem to work too well to recharge a mobile phone. It seems they will only raise the level of my phone 15% when the phone battery is rated at 2400mAh.