SunPass Customers:
SunPass customers using a rental vehicle may have their tolls applied to their SunPass account. Customers with SunPass Portable transponders may use their transponders in their rental vehicles. (It is strongly recommended that SunPass customers access their online SunPass accounts or contact the SunPass Customer Service Center to add the rental vehicle subscription to their account for the rental period.) Please remember to remove your transponder upon return of the vehicle.
SunPass customers that do not have their transponders in the vehicle are subject to the tolls, fees and charges set forth below and in their rental agreements.

Have a look here for rental car info: https://www.sunpass.com/rentalcar
and here for info on the transponders, including installation/placement: https://www.sunpass.com/howSunPassWorks

You can buy a SunPass transponder at a few different chains, including Publix, Sedano's, and CVS.