I doubt you can use the
DCL buses from Canaveral. When we've taken the bus, your ticket info is on your room key. I don't think they do same day sales. The best chance is take a bus from Carnival terminal to MCO. Before you cruise, put in dummy flight info with your WDW reservation to match when you get to MCO from Carnival (say 10am). Then take your bags down to the Magical Express counter at MCO to go to WDW. It is probably a lot easier than it sounds. Many people use the dummy flight info for ME to get a free ride to the airport and catch an earlier DCL bus.