Computer powers supplies are readily available and make a good 12 volt or 5 volt power supply. The only thing is when they are not connected to a computer you need a way to turn them on. It's really very simple. This is an ATX Power Supply for a typical modern computer.
Step 1: Jumping Pins
Looking down on the main ATX connector with the locking tab facing up. Jump the green and black wire, they are the pins 3 and 4 counting from the right.
I spliced and soldered in a switch to make it convenient to turn on and off.
Step 2: Video of the Process
Here is a video of the process for those who like to view it.