Best method is to get a 12v PC Power supply , they come available in various wattages , so try to work out what you'll be running and ensure you get one that will be strong enough.
If you get a mini-itx size motherboard with a few basic peripherals 80 - 200w should be fine.
Perferably get one with a built in power controller that will start & shut down / suspend the PC automatically when the ignition switches on and off
Last I was looking the closest supplier was in NZ
http://www.itx.co.nz/