I went right to the web site and ordered the transponder. It is interchangeable if you own multiple cars as long as you log onto your account and register all your vehicles. I believe that you get a discount when going through the toll booths as well. I have $25 put onto my account and when the account gets under $10, they hit my charge card for another $25. Sun pass also works when you park at MCO as well.
Check out their web site at
Florida's Turnpike - The Less Stressway™ and see if it is something that will work for you.