The nearest Airport is 90km away (Coimbatore). From there, you could rent a car (rented cars have to be dropped back) or hire a taxi to Coonoor (Taxis charge roughly $30/- or Rs.2,000 for a hatchback or sedan). I can help with getting a taxi from or to the airport so feel free to ask.

The best way to get here if you're travelling from Bangalore, Coimbatore or other relatively nearby cities is to drive. The roads are good, views are pretty nice and it gives you a whole lot more flexibility and independence.

There are also air-conditioned buses that you could take.