Hotel/B&B south of Sienna,

You might want to pick a town and look for accommodations there on Booking.com.
Or, go into GoogleMaps.com for the area and magnify the map by hitting Control--"+++". Little icons will pop up--many of which are for small hotels and B&B's. Click on their icons and it's often going to put you into their direct websites.
Virtually every farm south of Florence has apartments and rooms for rent for a secondary source of income. I'm talking hundreds and hundreds of great places to stay.

Tell us the villages you would like to visit. That may help get recommendations for that particular part of Tuscany.

I can totally recommend Podere Cunina, fantastic agriturismo with the toscan dream view, great people, perfect appartment. 5 minutes from Buonconvento.
I was there for 6 nights in june.