ferry from HKIA to Shenzhen

Sorry, I can't find schedule of last ferry at 21.45. Only the one at 21.20 is shown.
If the last one at 21.45 is confirmed, when out of the plan, where do V head to for ferry tickets purchase, Please? And, would it be any advance ticket purchase possible, or there would be no problem in getting tickets on that very evening?
Thanks a lot