Buying lots of soft drink or other effervescent drinks can be expensive (not to mention hard on the environment, with all the plastic bottles). You could get a soda-making machine for a couple of hundred bucks, but a cheaper and more satisfying alternative is to make your own.
Randy Stoltzfus on the Etsy Blog offers an in-depth how-to for creating your own seltzer maker. Besides the CO2 tank and regulator (available at a dry ice or welding gas supply company) and some odd auto parts store items, you'll just need some basic tools. The photos in this step-by-step make the project look pretty easy to accomplish.