One crutch (like when you have a broken leg) i used my sisters so it was free but you can get one a t salvation army for like 10 or less.
high powered spring-mine was 3.50 from http://www.mcmaster.com.
Make sure that the spring is slightly smaller than the crutch so that it can go inside, but you still get maximum power.
Knife (or drumstick if you want to play it safe).. i got mine from china town for 10.
NOTE- in the picture im using a wimpy spring from an old airsoft gun.. didnt get too much distance but the spring i ordered from the link above works beautifully.