Sure you could go buy a USB charger off the shelf, but what fun would that be? Try making one yourself with a little help from a 9V battery and a few extra components.
This simple hack on a standard 9V battery is an easy one to undertake. It's pretty straight forward, and all it requires is a quick solder to finish things up. It will give you power on the go and a sense of accomplishment for the day upon completing it. Hit up tech weblog anythingbutipod for the simple diagram to make your own.