^Apple doesn't upgrade any parts of the MBA after it's sold. They'll only open it up if it needs repairs.
If you open it yourself to add an SSD this like one, you'll void the warranty. They're also pretty expensive.
http://www.ebay.com/itm/APPLE-OEM-SSD-HARD-DRIVE-UPGRADE-256GB-PCIe-FOR-2013-MACBOOK-AIR-AND-MACBOOK-PRO-/151309508310?pt=US_Solid_State_Drives&hash=item233abfded6
You could just get a Transcend JetDrive SD card -- they're designed to sit almost flush with the side of the Air when plugged in.
Amazon.com: Transcend JetDrive Lite 130 128GB Storage Expansion Card for 13-Inch Macbook Air (TS128GJDL130): Computers & Accessories