This isn't a project that I have made but it looks really good and there are complete instructions in the Art of the Catapult book. Would be a great project.
PVC is a type of plastic piping that is used in the building industry for plumbing. It is inexpensive and easy to use but you do have know a little bit about how to use it and how to assemble pieces together. You can get PVC and all the ancillary supplies for it at any hardware or home improvement store.