Natively, Apple offers solid state drives ranging from 64GB to 256GB in the new MacBooks Air, with an option to reach up to 512GB in top-of-the-line models.
Now, the Other World Computing came up with your alternative.
The OWC Mercury Aura Pro line offers SSDs even faster than Apple's, reaching a performance of 500MB / s.
There are four versions available 120GB (US $ 168), 180GB (US $ 225), 240GB (US $ 320) and 480GB (US $ 645), all with a three-year warranty and with OWC's DuraWrite technology, for greater data reliability and durability. The package even includes the tools needed to install the new drive.