Theo is right.
www.sunpass.com. You can have it mailed to you (mine took 4 days) or buy it in Florida. Just mount it on your car and you are good to go. If you are renting, it is a little more complicated. You must add the rental info (plate #, make and model) to your account so that your sunpass is recognized. You do that by internet or 800#.
I leave Friday - - with my new sunpass!!! Good luck...