I have never seen tickets for these ferries sold online.

Typically, the tickets are sold by travel agencies at the harbor on Greek islands like Kos, Rhodes, or Samos. Generally, these ferries are smaller boats, and make only one or two sailings a day.

Here is a webpage that will give you some additional information.

http://www.turkeytravelplanner.com/t...ndFerries.html

Best of luck.