One of the only people that do the air thermostats in Europe is Skandix.de at Euro 9.50 from whom I got a couple but, you do have to take account of their excessive delivery costs on top.
If you do want to separate the diverter from the air filter box you will need to take the box out of the car as drifting out the diverter assembly from the box is a bit of a buggxr.
You have to tap two plastic holding lugs in (they may well snap off) and, then with the whole assembly mounted at a suitable angle in a vice, or the like, with a decent heavy hammer and, a wooden drift , gently thwack parts of the diverter near the joint to separate the two parts. Once it starts to move it's a doddle.
If the plastic holding lugs have snapped off you will have to drill a couple of holes near the joint to insert a couple of self-tapping screws to hold the diverter in place and, wrap a length of duck tape over the holes at the original securing lugs.
It's not too difficult really.
Bob
