There is one taxi service in the area, but it doesn't work like a taxi stand, you need to call them and book ahead of time.
I would suggest getting in touch, asking for price estimates and, if you want to go ahead, booking a date and time so that you know you will have the transport. Don't wait until the last minute. They are called Chianti Taxi -
http://www.chiantitaxi.com/
The alternative and one you should consider, is rather than take a bus from Florence to Panzano and then arrange a taxi, is to get a tour directly from Florence. You can either for go a private tour (such as
this one) or join a small or larger tour group (shared costs) that will take you directly from Florence into Chianti to several wine estates for wine tastings. Check out some tours here:
http://tours.discovertuscany.com/en/8357/Chianti-tours/Wine-Tasting-and-Winery-Tours/d26111-g6-c21