Otherwise, the two largest areas where you can get a rental car would be Florence (to the north) and Arezzo (to the south) both of which are easily reached by train from wither San Giovanni or Montevarchi.
Since to get to Cavriglia you'll likely need a car, I suggest as you make your way there and pass either through Arezzo or Florence to rent there and proceed. You can read some tips on driving and car rental here, plus use the search box to find the best deals on car rentals: https://www.discovertuscany.com/tuscany-car-rental/