I use a computer power supply. I got a piece of phenol board from work and put brass screws with wingnuts through and the power wires connected on the backside. I have a voltmeter I bought for 2$ on ebay right from china mounted in an electrical junction box. I also have the voltages wired to blade terminals for another connection option and voltage probes. I had gotten a lot of #10 ring terminals so making up leads is free if I use the wire I have gotten from psu's and cars. The bright light is 100 5mm white leds on a prototype board with 2 pin power connector, all parts from tayda.
you take the green wire from the 20 or 24 pin connector and ground it to turn it on assuming it is atx compliant. most psu's within the last 10 years not aimed at servers and not made by dell are atx compliant. you will get high current outputs of 3.3, 5, and 12volts as well as -12, and -5 volts at lower currents. you can also get intermediate voltages by moving ground and positive between the different voltages. basically just take your voltmeter and test different options. keep in mind that the negative voltages only supply up to 2amps on most psu's. atx spec requires the voltage to stay the same within 40mv.
some psu's require minimum loads on the 3 positive legs but most have internal loads to handle this. an easy fix is to just hook an old optical drive to it, most that need a load will be satisfied. keep in mind that large changes in load, around 6-8 amps may shut it down as it may think its a fault condition.