When replacing the screen in the air it comes as 1 whole unit, you have to replace the whole lid.
I took my air for a new screen, it had a blemish on it, and the guy said that because its all fixed they will have to put a whole new lid on it
so i dont think you will be able to replace it unless you get a whole part, i think they are about 280 pounds in the uk and apple charged about 24 for fitting it, thats what it said on my invoice, but i didnt have to pay because mine was still under warranty