First, almost all the car rental companies are on the airport at MCO (with a couple of the real
budget places needing a shuttle)
You can try checking prices directly on the car rental companies websites (alamo.com budget.com dollar.com etc.) or use a search engine like carrentalsavers.com to search several at once to see who has the best pricing (I recommend booking on the actual car rental company's website after you use the search engine to get an idea who has the best pricing first)
When you do your search on the rental car website (IE:
Alamo.com) the Orlando airport code is MCO, uncheck the box that says "return to the same location" . This will give you a second box where you can type in Port Canaveral, then pick the location from the list that pops up (probably something like Cape Canaveral-Port Canaveral. Put in your dates and times (note that you will be charged the same price whether you book it for 2 hours or the full 24 hours so book to give yourself enough time to make the trip comfortably.
Most rental companies have a shuttle that will take you to the port (and one that will pick you up at the port and take you to the rental location when you return)
For the return trip use Port Canaveral again as your pickup location, for the drop off location start typing in Disneyworld and a drop down will appear with possible locations (ie: Disney Car Care Center, Disney Dolphin Resort etc). Pick the location that works for you, Car Care Center is popular (near MK) and offers a shuttle back to you resort for drop off or pickup.
Plug in you dates and times and see what you come up with (prices change often so don't get discouraged if they seem high at first).
Book what you are comfortable with and check back regularly, if prices drop book the new lower price then cancel the old reservation.
If you are finding the return prices to the CCC are high, try checking out returning to the airport (MCO). If the airport return is a lot lower, I would consider just returning the car directly to the airport and forgo the DME ride back to the airport.
If you are flying on a "participating airline" you can still use RAC and check-in your luggage at the resort (don't have to ride DME)