Advance Purchase of Ferry Tickets from Shekou to HKIA

Hi, I would like to purchase 3 ADVANCE ferry tickets --online-- to travel from Shekou Port to HKIA (and check in at the Ferry Terminal in Shekou). Where can I do this. I would like to buy the tickets now because we leave on the first day of Chinese New Year and I don't want to miss the ferry if there are crowds at the Terminal. Thank you for your kind reply.