Hitchhiking in Greece is rare nowadays, but not so uncommon in islands and mountains (between small villages). As another commenter said, there's a popular belief amongst Greeks that it is illegal, but as far as I know this isn't true (but might had been true for a period in the past, hence the myth).
Turkish truck drivers are mostly found in Northern ...
From my experience it is better to book in advance. Tickets are not impossible to find on the day of the trip but people usually book in advance because the trains might be full. If you choose to book in advance, you are committing to a specific day and time.
You can book your tickets from the TRAINOSE website: