I have rented three houses / apartments in Tuscany. We used three different agents. Some of the search engines on sites are better than others. Take a look at my webpage (non-commercial!) for some suggestions: http://francesw.tripod.com/italy.html

We found that we were happiest with locations in towns so that we could stroll after dinner. However agriturismo is very popular with many people and might be more suitable if you are a larger group.

Frances Toronto, Canada