Where can I find speed and thrust data for airplanes during the approach? While trying to gather information on fuel consumption during the approach I have found this equation : 

$$f=C_{f1}\cdot (1+V_{TAS}/C_{f2})\cdot T$$  (on this FAA paper)


$f$ is the fuel flow
$T$ is the thrust acting parallel to the aircraft velocity vector
$C_{f1}$ and $C_{f2}$ are the first and second thrust-specific fuel consumption coefficient 
$V_{TAS}$ is the true air speed


From what I have understood Cf1 and Cf2 are constant and their values can be found (all the documents regarding these issues are 100 pages papers so I have not read everything yet).

I would like to plot the fuel flow for different airspeeds and thrusts. What are the typical values during the approach ?
I would like to have these data for several flights during the last 20 or 25 minutes of the flight.

What I am looking for is exactly what flightaware.com provides except that they use ground speed (an exemple here). How can I convert to TAS ? 
Also they don't provide anything on thrust. Do you know where I can find thrust data ?
All you need is collected, though not published, in airlines' Flight Data Monitoring (FDM).
Airlines collect a load of parameters for training and statistical purposes. If you can contact an airline, tell them what you're doing and make it sound like you could improve fuel economy, there's a good chance you can get some of the data.
Another place to seek would be aircraft manufacturers like Boeing or Airbus.