With the World Championships happening at that time I'm sure a lot of fans have already booked up rental bikes, especially in Firenze.

I have rented from Chrono Bikes in Lucca (about 80km west of Florence). Paladino, the owner, is awesome and will certainly help you if he can. I rented a carbon Viner from him for 100 euros/week. Good luck - but act as quickly as you can.