Mengmeng,
I'm a working mom too. We wanted to hire a babysitter, if not a domestic helper, but we didn't know where to look for. So we started off with HK women development association, where they have trained mothers who can babysit at their own home. But there is a waitlist now with newborns and they cannot confirm for a babysitter until two weeks prior to service so it will be too much of a rush if things don't work out. We end up putting flyers in our neighbors' mailbox and a few phone calls turned in. At last, we got lucky and quickly arranged with one of our neighbors just within a week.
If you want to hire a domestic helper, it often takes very long. If the helper comes from abroad, it take 3-4 months to arrive. Even those that are working in HK and finishing their contract, they still need to fly home for a week or two before returning so you better act quickly. Hope that helps.