I would rather take the opportunity to visit some local wineries and buy directly from the producer. I am not an expert on wineries in that area but I love Fanini's wines, their winery is in Petrignano del lago
http://www.cantinafanini.it/
Goretti is another winery not far from the lake that has reasonable wines http://www.vinigoretti.com/ The "arringatore" is particularly good.
If you rather have a variety of wines from the region but want to keep a low budget, then drive to Ponte San Giovanni at the Ipercoop supermarket. Here you will find a variety of wines incomparable to any other supermarkets in the province at very good prices. It's also a good place to buy anything you might need for such a large group of people.