Have any old disposable cameras (with a flash) lying around? Here is a cheap-o button that can be made from the button used to charge the flash and a cork.
1) Find a cork and cut it 1/2 of the way down as depicted. Make sure to cut it 'hot dog style' not 'hamburger style'. Aka: longitudinally.
2) Break apart the disposable camera and pull out the PCB board. Make sure not to discharge the capacitor (big black component on PCB board) with your finger when doing so... ouch. Looks like using the capacitor for a "taser" has been explored here on Instructables, so I will not go into that any further :).
3) Cut the button out from the PCB board while making sure to preserve the button and the two solder points you will need to attach wire.
4) Heat and remove any old components attached to the on board leads coming from the button. Solder your wire connectors in the old components' through holes.
5) Slip the button into the slice you made in the cork.
6) From here, you can cover the button in duct tape if so desired (not pictured). I simply placed a band of Velcro around the end of the cork to keep the button in place (pictured). The tightness of this Velcro band can be adjusted to hold the button in the "on" position if so desired.