I would try running canned tomatoes (crushed or whole) through a blender or food processor. You may then need to cook it down a bit to get the slightly gloopy consistency of passata. Or just use a jar of Marinara, which I think is pretty similar.
BTW I’m surprised that it’s not widely available in a country where Italian cooking is so widespread and influential.
Maybe try making fresh:
http://www.mylittleitaliankitchen.com/the-queen-of-pasta-sauces-passata-di-pomodoro/
