Tijuca Forest-Rio de Janeiro Questions

We are planning a visit to Rio this spring. I have been told that Tijuca Forest (which provides the city water supply), is much larger than the park-you ca go miles into th rear, and thids area has all kinds of interesting stuff (waterfalls, lakes, and other stuff). How do you find a qualified guide to take you into the back country? All of the regular toursit offcies in Rio just do the regular park tour-how do I find a guide who really knows the palce?