Most USB car chargers do the job,but try to get a high capacity one,especially if you want it to charge other devices too. I bought one that charge two phones or other USB devices at the same time in a local car part store.
"Dualtrip, the ultimate charger, features two, side–by–side high–speed USB ports capable of simultaneously charging two smartphones or tablets at full speed. Its elongated shape is engineered to stay cool and safely prevent overheating while outputting 2.4 amps from each port, making it the fastest way to charge your gadgets."