If you have not booked yet your tickets, once in Florence, you still have multiple options to make sure not to miss the most famous museums (Uffizi, Accademia, Pitti and others):
Ticket offices of the State Museums (except the Accademia Gallery) during their opening hours.
Firenze Musei ticket office are located at the Orsanmichele Church in via dei Calzaiuoli, Opera Your Preview in via Por Santa Maria 13/r and My Accademia bookshop located near the Galleria dell'Accademia, via Ricasoli 105r. Here are their opening hours: Orsanmichele - Mon,Thu,Fri 9am-3.20pm and Tue,Wed,Sat 9am-4.20pm; Opera Your Preview - avery day 10am-6pm; My Accademia - Tue-Sun 8.45am-5.30pm.
The extra charge to book admission to the Uffizi Gallery and the Accademia Gallery is € 4.00 per person per museum. For the other State Museums the cost is € 3.00 per person per museum. This is a pre-sale, so tickets have to be paid for at the time of booking.
If you are not able to book in any of the above-mentioned ways, another possibility is to go to the entrance of the Uffizi or the Accademia half an hour before opening time (8:15am) so that you will be the first to enter as soon as the museum opens.
In summer, you could also visit these museums on the dates they exceptionally open by night.
Remember that during high season, the queues at the entrance to the Uffizi or the Accademia may reach three hours of waiting time.
Santa Maria del Fiore (Duomo) Buy your tickets and book the Dome (Cupola) at one of the following ticket offices: piazza del Duomo 14a (right transept side) and piazza San Giovanni 7 (North Baptistery side). A totem for automatic ticketing is available.