Well if your set on the ac to dc power then here's the exact place i got mine,
110V 120V 220V 240V AC to DC Converter 12V 30A | eBay
then I run that to a 4 channel amp that has equalizers, then off of it i have 2 auxiliary cords running into a 2 to 1 audio cable that plugs into my laptop.
I only did this for the fact that the only thing I didn't already have was that 25$ power converter. Otherwise if you're starting from scratch just get a receiver...
And why you see people using a computer PSU is because it converts wall power into 12 volts. A car headunit only needs, a ground, 12v power, and remote turn on ( also 12v ). After that it's a matter of either running off headunit power, or hook up an amplifier to it.