12 volt plug for my motorhome.

Continental motorhomes (mine is a Rapido) usually are delivered with a mixture of UK three pin electric sockets and adjacent to them the continental 12 volt socket. Does anyone know where you can buy the 12 volt plugs with a USB fitting so I can, for instance, charge my smart phone? Please note: it is a smaller size than the cigarette lighter socket fitting - I have plenty of those!