The info is not available outside of Southwest's internal system, and that's exactly the way they want it. If you could easily comparison shop Southwest's fares with other airlines, you would realize that they're rarely the cheapest option.
ExpertFlyer does have the list of published fares for each route, but since Southwest does not make the fare bucket availability info available outside their internal systems, you can't use the list of published fares to determine the fare you can get on any particular flight.