I don't know, but if it's just one or a few you could try taking it in to an Apple store if one is near you.
A few years ago one of the arrow keys started falling off (the plastic clip-on bit on the back of the key cap had broken.)
At first the genius is talking like I'd need to replace the entire keyboard. But after talking to him for a few minutes about any possible alternatives, he say, "wait a minute," takes it into the back and 2 mintues later comes out with a new key snapped in place. Apparently he had a keybaord in the back he could canabalize the key cap off of.
(On the other hand, I'm not even sure if MBA key caps can be popped on and off like that.)