how can i find the part that directs the hot air flow in my vauxhall corsa?

The part (cog)that controls the temperature in the car has snapped off of the heater system. I was told that the car needs a whole new heater system as the small pin that holds the cog in place has snapped off. The only problem is that i've now been informed that it's proving difficult to get hold of this cog that directs the amount of heat that comes through. At the moment the car only blows freezing cold air. The guys were able to jam the heating on to high to keep me going, but i'm back to cold air again!!!
Does anyone know what this part is called and how I can get hold of it?
Help, please!!!!!!!!!