Actually finding a domestic helper is very cheap and easy.
You could find the candidates from askexpat (website) or supermarket noticebord. Get their telephon number (most of them are those who finish contact with experience) and arrange for interview.
If you decide to hire them, they will get the working visa by themselves.The are genius.
What I paid was the admin less which is less that HKD 500. For those are not fisrt time work in Hong Kong need not go for medical check up. You need to buy insurance for them (cost range: HKD 300 -HKD 700/ per year). Those helper who finished contract is better as they well trained.
You also can look for those have experience in looking after new born baby.It is important to find someone with experience to look after the new born baby (those newly come from Philippines/Indonesia will drive you crazy- for example, they do not check the temp. of water before put the baby into the tub;they do not know that we should not shake the young baby and so on).
Hope you could find a helpful helper.