Go to Vodafone.it and you can search for stores. Prepaid plans are basically for 30 days and cover all Italy so where you start/end shouldn't matter. You'll have to purchase a SIM which usually comes with a nominal credit and then purchase a prepaid plan. Just make sure that it is set up to not automatically renew (only an issue if you want to use the SIM again as the auto renew will wipe out any carry over balance).