Seeing The Last Supper on the Web
Before you go, you should have a little information on the church the Last Supper is in, and some information on the work itself. Arounder Milano has this information, and has a very detailed scan of The Last Supper available online. See The Last Supper by Leonardo da Vinci.
Now the the Last Supper can be viewed online at 16 billion pixels by Italian imaging firm HAL9000. You can really get a close look, zooming in on key details you'd like to see. Note: it may take some time to load.
Buying Tickets for The Last Supper
The debut of the Da Vinci Code movie brought out the conspiracy seekers in droves. Only 20-25 people can see the work at one time, for a maximum of 15 minutes. Only if people don't show up will you be able to stand in line and hope to get a ticket.
Last Supper tickets can be ordered online from Select Italy. Try to aim for three months advance purchase. For details: Museum Tickets for The Last Supper - "Il Cenacolo" Milan Italy. There is a surcharge getting tickets mailed to you in the US, of course.
Cenacolo Vinciano in Italy takes reservation by phone or online. Instructions are in English but remember that if you call the phone number, Italian time is 6 hours ahead of US East Coast time.
If you have a hotel already, you might contact them and see if they can get you tickets.

