Buy Ferry tickets online and pick them up at Athens Airport
The most convenient way to get your ferry tickets, is to buy them online from Aktina Travel, and get the paper tickets when you arrive at Athens Airport from their offices. Find thems at the Arrival Halls between the Gates 2 & 3 (you cannot miss it, its a piece of cake to find it). Alternatively you can get them from their offices in Athens Center, in Syntagma Square (The most central square in athens).
Have also in mind that ferry ticket prices are not like airplane tickets. The prices are fixed and they do not change like stockmarket. Of course you can find offers. Using Aktina Travel website you find the fixed prices. (I have seen in some websites, mainly not from greece, selling them more expensive)