You do need much more than 10A. Look around for a server power supply, the HP ones can deliver at least 50A and you can get them for just 20 to 40 dollar. You only need to solder a bridge or a resistor and 2 plugs to it.

Here a list of instructions for all kinds of server PSU's How to convert Server Power Supplies - RC Groups

But why not a simple battery? Or if you have a real car just make some extension leasd to connect your box to the car battery.