Eu comprei destes:
http://www.motoclasse.com/index.php?mod=catalog&action=viewProduct&product_id=2461#.Vso8JfmzIy0
Tanbem tens deste genero:
http://www.ebay.com/itm/Waterproof-12V-Dual-USB-Power-Adapter-Charger-Socket-Outlet-for-Motorcycle-Car-/301786289834?hash=item4643dd56aa:g:iWkAAOSw4iRWE2Uf