Across the entire website, there is a box right below the main photo - the 3rd tab has a link for booking museum tickets, including San Marco.
You can also do so over the phone +39-055-294-883 since you don't pay ahead of time for the tickets.
San Marco is not a super busy museum so most of the time you can walk up and buy tickets on the spot. You don't need to book ahead for a group of 15 people, there is no group discount in any case. Buying on the spot you won't have to pay the extra 3 euro fee for prebooking ;-)