Yeah go to Sino Centre in Mongkok--not too far from the station but you do have to walk a bit, and the sign is in Chinese. In Sino Centre go to the basement (like the B1 floors) and all the CD shops and CD merchandise are there. They sell posters and concert merchandise, so you can probably find stuff like the AT2008 keychain.
No guarantees though--personally I've never shopped there for Ayu stuff. But apparently they have lots according to other Ayu fans.