You could build it with any power supply from 6 to 12 volts if you do not have a broken drill laying around.
This is a total hardware hack. I know it looks like crap. We were having a party and I wanted a prop that would scare people. I threw it together in about 2 hours. I've spent three times longer documenting the build than I did actually building it.
I have a really well stocked junk collection, so everything that I used to build this was actually laying around the shop before I started building it. The design is very flexible and will work with whatever junk you have.
Step 1: The power source
I relocated the PWM power transistor to the side of the drill handle, because there was no room for it inside after the top of the handle broke off.
I attached two heavy (16 GA) wires to the power output of the drill speed control.