Search for an area that is central to the destinations you want to visit and within reasonable driving distance from an entrance to the motorways (some Agriturismi are in the boonies, even requiring driving on dirt roads). So choose your agriturismo carefully using Google Maps to see exact location and also to see the general area you want to be in. If your destinations are Orvieto, Assisi, Siena, Florence, you want to stay not too far from the Bettolle area where the Autostrada A1 and the Raccordo Siena-Perugia meet. From there you are within one hour from any of the above mentioned destinations.
Someone will suggest some names where they stayed in this area (I never have stayed in this area). But otherwise you can use the following websites for some searches:
Also take a look at this German website below. (click on the British flag for English):
Be aware that most agriturismi require a 3-4 nights minimum and many even a min. of 1 week stay especially in summer.