The only official website to purchase your tickets is this one:
https://hhticket.gr/tap_b2c_new/english/tap.exe?PM=P1N
Tickets for the summer period, starting April 1st, are not yet available. They will be available from mid-March (approximately).
For the Acropolis, you will need to specify a date and time slot for your visit and arrive on time at the entrance.
The combined ticket was discontinued last year. All other offers from third-party companies are generally not worthwhile and are obviously more expensive than purchasing individual tickets for each site on the official website. (I am only referring to combined tickets in all their forms, not private or group tours with an official guide.)
The Parthenon is one of the monuments on the Acropolis.
Which museum are you referring to? The Acropolis Museum is privately managed and has its own website:
https://www.theacropolismuseum.gr/en