Of course, you have to go to a notarized law firm in Hong Kong. Of course, you can consult these places by telephone and apply for notarization without criminal record by telephone. Personally, these institutions should have opened the service channel of online application.
In addition, you can seek help from the Chinese mainland service agencies for Hong Kong people, and maybe they will introduce an intermediary agency to do it for you. The mainland charges about 300, but Hong Kong won't charge too much.