Cross border bus at HK airport to Guangzhou

Where would I buy the fare to this while I am at the airport? Would I need to stay in the security area and go directly to the cross border bus? I do not know when the first cross border bus leaves the airport but I would want to take that one. Also, if I decided to wait it out till next morning at the HK airport for the first bus, is it possible or allowed for me to do this. Since I land at 1030 and buy the time I get luggage it might be around midnight.
ThAnks to all for all the invaluable info!