Tickets of sites and museums
When we travel we always are anxious if we are going to fine tickets and where to buy them. In Greece, you may check the official site of the Greek Ministry of Culture which is the following.
https://hhticket.gr/tap_b2c_new/english/tap.exe?PM=P1N
Tickets should be bought online before your tour.
You choose Region of Attica and Central Greece and then you book them one month prior to your visit (they are available one month in advance).
As for the Acropolis museum you can always buy them on the spot with your guide skiping lines.