To power my carputer I didn't want to use an inverter to convert my car's 12V DC to AC and then back down to DC again with a normal ATX power supply. A guy that goes by the name hoply sells kits to build your own DC-DC power supply. I picked up his last small form factor kit and built my own power supply. It works great, takes up little space, and provides good clean voltages. If your not good at soldering, OPUS Solutions makes a sweet DC-DC power supply.:

ATX, 6-30V wide input range
- Intelligent shutdown controller
- ON/OFF motherboard control
- Survives vehicle engine cranks
- Battery deep discharge prevention
- High efficiency, 90 watts output
- Works with car batteries
- "Anti-Thump" Amplifier remote control
- Automotive fuse (mini-blade)
- Works with all VIA mini_ITX  AMD and Intel ATX&micro-atx M/B
- VIA, P4 and AMD CPU support