One like this but with 3 holes instead of 2 link
Most Helpful Guy
The third prong is just a ground lead. It's only there really for items with a metal case in order that the user does not receive a shock when handling a faulty device in which the power lead comes loose and contacts the metal casing. It's not needed for the operation of an electrical appliance but just a safety precaution when dealing with 120+ volts which you wouldn't be doing with this anyways. If things went horribly wrong you'd only be getting a shock from the battery which is harmless.
That item actually looks to be designed to accommodate plugs with the lead despite not offering the functionality of it by allowing you to overhang the third prong on the grooved side.
As far as I know that item is one of a kind.0