Midwest does exchange CO2 cylinders, but IN-STORE ONLY, as full cylinders cannot be shipped. Otherwise, just search for local Gas or Welding Supply companies, most will be able to exchange your tank for a full one.
For proper dispensing of nitrogenated Stout beers, such as Guinness, you'll need to use a mixed gas of Nitrogen and CO2. Inquire with the Gas and Welding Company whether they have 'Beer Gas' or a 'Guiness Gas Mix'. Generally these mixes are 75% Nitrogen and 25% CO2.