My husband recently advertised on the HKU web site (we live near the University maybe you could try Chinese U) for a Mandarin tutor. We got four students replying to the advert within 24 hours. The one we have hired is a second year student from the Mainland who speaks very good English, native Mandarin and good Cantonese (in fact amazing Cantonese given that she's learnt it since she arrived in Hong Kong). I was doubly happy because she is an education student and is happy to be somewhat innovative in her teaching methods. We pay $250 an hour.
Best wishes,
Barb