Where to buy Dandong to Incheon ferry tickets, in Beijing or other cities?

Hello, I will be in BJ and afterwards I will travel to Dandong to catch the ferry to Incheon in South Korea hopefully. Does anyone know if I can buy the tickets in BJ or in the city of Dandong itself, or do I have to go to the actual harbour itself to buy them, also how far in advance would I have to buy the ticket, would it be possible to buy them on the same day?

Thank you.