The Hellenic Heritage website (https://www.hh.gr/en/) which is run by the Greek Culture Ministry, has recently been launched and now offers comprehensive information on all the major and many minor museums and archaeological sites in Greece. It is way better than the old Culture Ministry site (odysseus.culture.gr), which was always a bit impenetrable.
At the moment you can buy tickets on the site for about 30 destinations - from April this will be expanded to over 100. Also, because it is government-run, you will not be ripped off by ticket touts if you buy your tickets here.
You can find it here: https://www.hh.gr/en/