Russel, the above recommendations are good, but you may want to try the original Jamacian Ska which later influenced the English bands listed above. Early Bob Marley is an excellent place to start. Simmer Down, which you can find on the Bob Marley & the Wailers Trenchdown Days, Birth of a Legend CD, is considered one of the best Ska songs ever written. The Skatalites, which was the house band performing on Simmer Down have several great albums out. If you can find it, pick up Stretching Out, 2 cds worth of live SKA with killer songs such as Latin Goes SKA, Guns of Navarone, Man in the Street, and Big Trombone. Although not quite ska, which is essentially fast reggae, The Best of Desmond Dekker is a wonderful album featuring the song Israelites. For a new band entrenched in the classical roots of Jamacian SKA, try Out of Nowhere by Hepcat. Hope this helps
