I'm happy with my gray Incipio Orion. It fits perfectly and provides good protection. Got it at Amazon for $29 although you can also get it direct from Incipio for the same price.
I have an Incase Neoprene Sleeve for mine. It's good for light protection and has held up better than I expected without signs of stretching after putting in and taking out my Air a couple of times every day for months now...