From the FAQ:
It must be a DC power supply. It must supply direct current as output, not alternating current.
The connector jack must have the center pin as positive (+) connection, and the outer sleeve as the negative connection.
The connector jack must have a 2.1mm center pin and a 5.5mm outside diameter. Make sure it's the correct size, or you might damage the car/home sensing switch by forcing the jack into the player.
The voltage can be in the range of about 12v-16v DC, the exact voltage doesn't matter much.
If you search, you can find many options on Amazon, like this
. Be aware that the player does do some things differently in car mode versus house mode. I believe that in hijack you can force it to car mode. I would also use it with a power source that is ignition switched if possible or you might accidentally forget to unplug it and return to your vehicle to find a dead battery.
I used to carry one of these in my empeg bag at all times for when I used rental cars (along with an aux cable and a fm transmitter). Never had a problem.