The painting of the Last Supper by da Vinci is sold out months in advance in high season. I never plan ahead, but I did want to see the Last Supper so I had to figure something out. Sometimes you can buy a ticket off of a tour guide at the entrance if they have an extra one. The day I was there this tactic wasn’t working so I went to a bus tour operator because some tours include the Last Supper. Well of course they were sold out too, but I squeezed onto the waiting list and luckily just made it onto the tour.
Warning: this video is boring so don’t watch it unless you are in Milano and can’t otherwise see the Last Supper. ;-)