You can get to Radda from Florence and Siena by bus. Unfortunately the service is limited.
The company ACV – Autolinee Chianti Valdarno, by Busitalia buses, has a direct connection from Florence to Radda, Gaiole and Greve. The ride takes about an hour and a half and costs about 4€. Departures from Florence take place from different points of the city. The service is limited.
To download the pdf timetables check the schedule Florence – Gaiole in Chianti (365 direct bus)
Versione italiana: Firenze – Radda in Chianti