We've just returned from a 4-day stay in HK as part of our honeymoon and most of the tailors I saw were in Kowloon, just a short hop on the ferry from Hong Kong Island where we were staying.
There are lots of tailor shops around in Kowloon (and probably also in Hong Kong Island) and we were constantly approached by people on the street offering cards for tailor shops nearby. If I remember correctly, the hotel brochure will probably also have information on tailors, so you could try asking advice from your hotel?
It was our first time in Hong Kong and we both loved it! A busy, hectic place and the shops stay open until 11pm - hurrah!
CMCR.