Unless you work in an environment where there is a lot of dust and debris, in my humble opinion, a screen protector wouldn't really be necessary.
If you are using the screen protector because you are worried about the keyboard touching the screen when you close the MacBook, two things:
1) The MacBook Air has "built-in" protection - the bezel is constructed in such a way so that they keys do not really heavily touch the screen (unless you put something heavy on the top of a closed MacBook Air, which should not be done).
2) You can always clean the screen using the microfiber cloth.
That being said, if you would still really like to have a screen protector, I saw the MegaGear Screen Protector for the MacBook Air (just search on Amazon), and it had pretty good reviews.
I hope that helps!