We will be arriving in Venice at the airport and staying at a hotel in the San Marco area. Where is the best place to buy a SIM card? We just need a phone to call within Italy and for an emergency if we need to call the United States. We have an unlocked phone that we can just insert a SIM card. We will not be using the phone much at all. We just want to have it "in case" we need to make a call within Italy or to the United Staes. Many thanks.