I have read that tests are available at the airport without an appointment. We plan to do that on arrival before our cruise in a couple of weeks to satisfy the 3 day rule (we are staying in Venice before the cruise). Also, there is a pharmacy in Venice nr St Mark's square that does them for a lower charge than the airport. You need an appointment, but I have seen people say they went to the pharmacy to set up an appointment and then returned for the test. The name is "Farmacia Lupo Coronato". It isn't open on Sundays. There may be a site at Piazzale Roma (where the people mover is located) but I am having trouble confirming this. I will come back and add if I find out anything more.