How about using a standard 350W ATX PSU ? I use one to power my inverter when I am using the carputer indoors. Just remember to ground pin 14 (PS_ON green wire) to turn on the power supply. Some ATX PSUs need an external load on +5V to regulate. If the +12V rail is not 12V +/- 5%, you may need to connect an old hard disk to present a dummy load on the +5V rail.