Buses 8, 80 and 33 travel the zone of the “viali” [boulevards]. From the central train station of Santa Maria Novella you can catch bus 31, 32 or 6. Get off at the Via della Colonna stop.
Various ATAF buses stop in Piazza San Marco : lines 6,7,10,11,17,20,25, 31, 32, 33, and the “C” electronic bus.
(Download the map of the ATAF bus lines in PDF format)