So far I used AutoEurope or their affiliate Kemwel for my rental needs in Italy every year.
www.autoeurope.com
www.kemwel.com
It's a broker that works with the well known major international rental companies (Avis, Budget, Europcar, Hertz, Sixt, etc.) and also some other European brands such as Autovia (an Italian company), Centauro (a Spanish company), Drivalia (a French company), Locauto (a former Italian company now part of Enterprise brands in Europe), Maggiore (a former Italian company now part of Avis brands in Europe), Noleggiare (an Italian company), SicilyByCar (an Italian company).
AutoEurope has recently changed ownership, and that has caused some changes in their extra additional zero deductible insurance coverage (the standard insurance coverage is always included in rentals in Italy). More information on that change is in the recent forum threads below:
https://community.ricksteves.com/travel-forum/italy/can-anyone-help-with-a-rome-car-rental-company-now
https://community.ricksteves.com/travel-forum/italy/bari-car-rental