You will need a power supply similar to the ones shown on this page
13.8 is the often the actual operating voltage of a 12 volt system while the batteries are being charged by the alternator.
If you use one like this you will also need an in line fuse. The tech page recommends 5 amp fuse.
I did not look, but you may find something from a security company. Those systems often use 12 volt, but I am not sure that you will get 5 amps. Often those systems only use 500 or so ma.
You may get good results by stopping by a car audio store. It is very common for people to buy power supplies to either bench test equipment, or use their car stereo in the garage.