Hi Maria, 
Interrail Passes are sold at the main (international) train stations in Europe; however, not in all countries, and not all Pass types may be available. As we only sell Passes online, we don't know which stations sell Interrail Passes or how much they cost. We recommend you ask at the train station for more information. If they are not selling them, I am afraid the only way to purchase the Greek Island Pass is online. 

A member of this community (see post below) has already been in touch with the train station and they don't seem to have it available, but if you are there anyway I would encourage you to double check it yourself as well.