For this project you will need some stuff:
-A Scooter handlebar, one with that button that you press and he come up again, or any tube with that button in it,
-Any pipe/tube, better be an aluminium/steel one, copper is too soft.
-A spring, i got mine from a car handbrake asembly, the apring that operates the handbrake button.
-And most important thing, the Blade. you can get any knife you want and cut it in half, or you can do this really professionaly and order some throwing knives from amazon or ebay.
when you have done this, you should hammer into the other end a round screw or anything round, so you can regulate the pressure of the spring,and get some better shots.