Pick a city you're going to and search for a farmacia.  They're privately owned in Italy so tend to have websites to drive business their way.  Check for a contact page with email, and ask if they have "citrato di sildenafil" available over the counter without a prescription, and that you're not trying to buy it over the internet but instead will be there in X-weeks.  If you can translate the email then do so before sending, but don't worry if you can't - many Italians can understand English or have someone on the store staff who can do so.