Nobody has taken the time or effort to construct or build a Rubber Band Gun, but they are quite common. It is all probably for the better because the low prices on rubber bands mean that available ammunition is nearly infinite. As always I do recommend purchasing your products in bulk when you have the opportunity. Maybe you don't need 200 bags of rubber bands, but they are cheap to begin with and only get cheaper when you buy them in large quantities.
Rubber bands in general come in a natural rubber or latex-free variety. If you have an allergic reaction to the latex rubber bands you need to get the latex-free version. The rest of us have a different dilemma. The all natural rubber bands are made from rubber that comes from the Para rubber tree and are biodegradable so after many many years they may start to fall apart or stick to the things they are holding together. On the other hand, the latex-free rubber bands come from 70% recycled polyurethane but aren't biodegradable so they'll be in your office or landfill for longer than you'll want to think about. So do the research and ultimately buy whatever you like.
Maybe you don't care if your rubber bands are actually made of rubber because you just want to use them to shoot pieces of small red plastic at your friends and family, and I can respect that. Just remember that if you build your own rubber band powered arsenal to bring to your office I won't be held responsible for the cold war that will inevitably occur.