All the ferry companies allow you to pick up your tickets directly at the port of departure at their central agency (they may not be inside the port).
During the booking process, you will be asked to choose how you will receive your tickets.

To pick up your tickets at the port before the departure of the ship, you should select "Pick up from the port" and the confirmation of your reservation will show relevant information.

Please note that in order to pick up your tickets you should have your id/passport and the reservation code.