Hi
There are a few threads already on here where various people recommend some agencies (I think a few people have been very pleased with Arrow or Target). You can also be asking people you meet for personal recommendations, if they know a helper who is looking for a position. Word of mouth tends to be very helpful.
I have been in HK for 13 years, and from what I have experienced and shared with friends as they went through the same thing, no matter how good someone's reference is from another family, they may not be the right fit for your family. A good reference is fantastic, a real PLUS, but consider things that are important to you. How do you seem to get along?
Good luck