Here is how you can make your own water balloon launcher. We use high strength surgical tubing unlike many of the launchers you can buy online that don't shoot very far. The cost of materials is about $18. This instructable covers only the launcher. See the Water Balloon Launch Pad for instructions for the frame.
Step 1: Materials
(1) small roll of duct tape
(2) 8 inch lengths of 1/2" vinyl tubing
(2) 6 foot lengths of latex rubber surgical tubing
The vinyl tubing can be purchased from a hardware store. The latex rubber tubing can be purchased online. One good source is McMaster part number 5234K75. It should be 3/32" wall thickness (not 1/16" thick). The surgical tubing sold at hardware stores is normally only 1/16" thick and not strong enough to launch a balloon 300 yards. If you are in a pinch and don't have time to order the thicker tubing, then you can double-up the 1/16" tubing, but that gets quite expensive.