移民美國的公證在香港可以去哪搞??

What kind of notary you are looking for?

You can try to contact Home Affairs Department and see if they can help you.

http://www.had.gov.hk/tc/public_services/public_enquiry_services/provided.htm

there are USA certificated lawyer in HK, you can call them to see what to do.
once you finished the document paper, you can send it back to China and tell your relatives bring it to China Embassy for certificate.