From Florence in Italy to Zagreb in Croatia there is a weekly bus from which is scheduled to run each Saturday trough out the year. The 620 km long trip bus trip has travel time of about 11 hours. In Padua the bus meets with buses coming from other parts of Italy only one of the buses continue to Zagreb, which means that you might have to change to one of the other buses there.
The timetable for the bus from Florence to Zagreb is here