First question is, are you only going to carry around the MBA or are you going to carry around books and other student paraphernalia as well? If books, etc. are coming with you, type in a search for 12" laptop backpacks. You'll need that tiny extra space for the books etc.
Now if you are only going to carry around your laptop, a wallet and phone and little else...second question: are you male or female? If male read no further 😉 I found the perfect backpack for my MBA...but it's a backpack "purse." This one happens to be quilted (and so cushions my Air nicely), and the choice of fabric were, well, various shades of paisley :-D
So, I've a purple paisley quilted backpack. But hey, it works! It's little and fits my MBA, wallet, iphone, the cord, sunglasses, keys and not much else, but then I don't need room for much else.
Got it from this place:
http://www.thequiltedmonkey.com/
This is a small store run by the owner, and I'm sure she'll send you a picture of said "backpack" if you e-mail her.