I bought my last one from Hong Tai, quite cheap, Cebu Pacific(only HK$700 exclud tax, I remember paying abt HK$1100 all inclusive), but once the ticket is confirmed, they will charge HK$150 if you need to change the date.
I direct order from Cebu Pacific Airline via internet, I think it is the cheaper. My maid vacation leaves from 22/12 to 10/01 2 ways only about $1660(included tax).
She said her friends also same date but need more than $2200.