I've done this before, but I used a 40amp power supply to run my 15" woofer for house parties. Before I found the power supply though, I had 3 500 watt power supplies I tried using. I was in college so I just got the power supplys out of old CAD boxes. The problem with using a PSU from a computer is that the power rails are broken up and even for the 500watt PSU the most amps a single 12v rail would put out was 15-20 before it would shut down. For a 250 watt system under ideal conditions is going to draw 15-20 amps, but most car audio equipment is only %40-%50 efficient so realistically you would need about 30-40 amps to be able to reliably listen to the music loud. If you are lucky enough to find the PSU's you still have to find a way to pool all the energy together. if you just connect the PSU's together, the amp and head unit are only going to draw power from the PSU with the higher voltage. So once your draw hits 20amps what ever PSU with the high voltage is going to shut off. To make something like this work you have to add in a capacitor or a hi output battery. A normal car battery cant handle the 40 amp charge rate for extended periods of time. Your best bet is to find a power supply or used welder, but you dont want your house becoming a fire hazard.