This is an instructible that doesn't work completely like the ones from the stroe but its alot cheaper!

Step 1: Get What You Need

you will need
1.a can
2.a drill
3.a balloon
4.a rubber band

Step 2: Drill a Hole in the Can

get the can and remove the lid if there is one.Now flip it upside down and
drill a hole about 3/4 inches big in the center.

Step 3: Put on the Balloon

strech the balloon over the top of the can where the lid would go.

Step 4: Put on the Rubber Band

now after putting on the balloon, take the rubberband and tightly rap it around until the
balloon is totally secure.

Step 5: Firing

now to shoot the air-zooka, pull back the balloon and let go. if the balloon breaks
then add a new one and if this doesn't work try to make the hole a bit bigger and if
that doesn't work than just go buy one.
If you feel like you could add a handle too.