Converter or cable to car power supply
Are you looking for something like this?



It has a cord that plugs into the wall (120 Volts) in your house and a cigarette lighter jack just like in the car where you can plug anything into that you could plug into the car. It also has binding posts if you prefer to connect the wires directly. it outputs 13.8 V (which is what you will find in a running car on the nominal 12 V wires).

This particular power supply is from Radio Shack.

They are available in various current (wattage) capacities depending on how much current your device requires.



The manual says that voltage input needs to be in the range  9 to 32V and maximum wattage is 47W. 
12V power supplies are common. Thee may be sold as 13.8V supplies as that is the maximum voltage that a car battery usually reaches in normal use. 

So a 12V, 4A power supply is marginally good enough and a 12V,5A or 12V,6A is better. More current capability does not hurt. This supply is rated at 15A and would be suitable and is not much dearer than a 3A supply. You may wish to check that the ripple output of such supplies is OK for your system. Should be OK but some have more ripple than others.