There are plenty of sources on the internet to buy a rubber band gun, and a few to get plans to make one (and most of those are intricate gatling gun affairs).

If you want to make something really elegant, with an interesting design, you need to go here. The web page is inscrutably Japanese, but look at it through Babblefish, and it will become slightly more clear. There is only one set of plans, but many of the designs look simple enough to do from the pictures.

My favorite is the Giza 66°. Very very cool.