12 volt power supply

I have a 1997 E36 BMW M3.. I am splicing some electronics into other wiring, but before I do I want to make sure it's right. Where would be the best place to find a positive 12 volt power supply that is on only when the ignition is on? I am at school so I don't have access to the ALLDATA at my work. I am wanting to place it around or near the center console display. I would appreciate it if anyone had some ideas for me.. Thanks

You can try using a Computer power supply and just connect it to an extentsion cord to power the device? Are you just looking to test it out?