Since none are listed on the internet, your best bet will be directly from Apple or an Authorized Service Provider.
MacRumors is reporting that Toshiba is now making their “Blade X-gale” series solid-state storage available to all. Available in 64GB, 128GB and 256GB sizes, these are the exact same sticks that the company supplies to Apple for the new MacBook Air, but now they are available to the mass market as well.
“Available now, the new drives are offered in capacities of 64-gigabyte (GB) (1), 128GB and 256GB, with a maximum sequential read speed of 220MB per second (MB/s) (2) and a maximum sequential write speed of 180MB/s,” the company announced via press release. “Ideally suited for integration into space-sensitive products, including tablet PCs, laptops, mini-mobile and netbook PCs, Toshiba's latest SSD offering helps these devices achieve a super slim profile.”
MacRumors notes that the blade-style storage are the same sizes currently offered by Apple’s MacBook Air, right down to using the exact same part numbers. That means that early adopters who jumped on board the entry-level $999 11.6-inch model with only 64GB now have a potential upgrade path -- although the website is quick to note that they have yet to confirm that a 256GB stick will fit in the smaller MacBook Air, particularly since Apple doesn’t offer that as an option either.
“We should note that we haven't yet been able to confirm that the 256GB part will actually fit into the 11" MacBook Air, as the 256GB part is slightly thicker than the 64GB and 128GB parts (3.7mm vs 2.2mm),” MacRumors noted.
Toshiba has yet to offer pricing for the new storage, but it’s good to know that choices exist at all, with Taiwanese company Photofast also offering their own SSD upgrade kit announced at the end of October.
Also Taiwanese company Photofast has announced the first 3rd party SSD storage upgrade kit for the new MacBook Air.