First the good news - Apple's custom SSD blade drive is a replaceable unit so its possible to swap-out.The bad news here is its too soon to say when 3rd party replacements will hit the market as this system is just too new.The other bit of bad news is largest SSD so far available is still very expensive and only 920GB in size. I would recommend you follow this site: Transcend they appear to be aggressive on offering options for Air & Retina systems.
I end with a bit of good news here: Intel and Micron’s 3D NAND promises SSDs “greater than 10TB” so in a few years we should start seeing lower cost SSD blades and larger ones as well.
Update (03/12/2016)
Good News! OWC has just released their SSD's for the Retina systems!
Here's the details: MacBook Pro with Retina 13"" & 15"" Late 2013 - Current"