Museum tickets

The Vatican has changed its ticket offerings this year, so it's possible last year's options (if any) are no longer viable. There are definitely no longer commercial tours able to enter the Vatican Museums before the latter open to the public.

I think the solution most people use if they can't buy The Last Supper tickets directly is to sign up for a tour that includes it. You may find the names of specific companies via the Search function here. Choose "Travel Forum" from the Filter by Type box, then narrow the results down to "1 year or newer". This is what I got by searching for The Last Supper and following those steps:

https://search.ricksteves.com/?button=&date_range=1y&filter=Travel+Forum&query=the+last+supper&utf8=%E2%9C%93

I would try to book tickets for the Vatican Museuns directly from the official site:

https://tickets.museivaticani.va/home/calendar/visit/Biglietti-Musei

Tickets are now unique to individuals - the names of those attending must be entered when booking.