I originally built this to power a small coil gun but it can be used for whatever you need it for.
It runs off 3.3 volts but it will run off of 2 "D" size batteries with a reduced voltage output and slower charge times. The parts can be purchased online for about $20.
I recommend running it off an old computer psu. The one I use to run it is rated to 16 Amps on the 3.3 volt line and it runs this fine.
For instructions to make the 3.3v power supply look Here
Step 1: Warning!
This energy is stored in capacitors and it will still have power in it even when unplugged.
It is capable of putting out several amps! The output can easily be deadly!
DO NOT BUILD THIS UNLESS YOU ARE FAMILIAR WITH THE SAFETY PRECAUTIONS FOR WORKING WITH HIGH VOLTAGE
You are responsible for anything you do with this.
You should make certain that the device is clearly marked
Here is the warning for this device
High Voltage Warning Sign - for gimp
High Voltage Warning Sign - for photoshop
Here is the font I used on the warning signs above.