Not sure what you mean by farmhouse. Do you want to stay on a working farm with the family? Or do you want to stay in a house on farmland? We used Tuscan House rentals for our stay in Italy in 2002. Our home away from home had three bedrooms. It was a self-contained house so we had it all to ourselves.

The house was smack in the middle of hay fields (the farmer was taking in the crop during our stay - heaven) which was located about 2 hours drive south of Siena. However, Zak the local agent for the rental agency lives in Siena so I'm sure he could find you something. Here's the site: http://www.tuscanhouse.com/

Happy travels. Lucy, Toronto