If you're gonna be using led's to test in the cluster, a plain old 9v battery from a smoke detector should work.
If you're gonna be using incandescent bulbs, I'd suggest getting a 12v dc wall wart power adapter from some old electronic device & using that for your power. As long as it has 1000mA or 1A of DC output, it should be good even when using incandescent bulbs.
It will work the same for led's, just need to check the polarity on the wires. I've made & used several of them over the years. They're cheap & handy to use & if it happens to fly across the shop & smash into a million pieces, I'm only out a few bucks
I'm way behind on getting more **** inserts made up, but I may have one set done