It's not that well advertised. Head out the airport, turn right and walk all the way down past the taxis. You can get a ticket from the small booth (y)
The last metro leaves the airport at 23:30. But the x95 bus runs all night (more or less every 20 mins) and drops you off at Syntagma. It'll take about an hour (compared to 40 mins on the metro) and costs €5.
I put this on a different thread but It's probably useful here too:
I think there is little chance of getting a ticket in the home end either*. Over here they require you to register for a fan membership and provide your Greek ID and social security number to purchase a ticket.
*The caveat...