You can get upgrade kits, not cheap though:
https://eshop.macsales.com/shop/ssd/...2013-2014-2015
Or eBay to find someone selling an OEM replacement for cheap (cheap doesn't seem to exist).
There must be adapter cards out there because there's eBay listings like this one:
http://www.ebay.ca/itm/256GB-SSD-for...-/221867260365
Which seems to be a normal m.2 SSD taped to an adapter. I think the 2013+ MBAs use PCIe SSDs not SATA ones like mine.
This MIGHT be the adapter, but it seems tough to find the right adapter and right SSD.
http://www.microsatacables.com/ngff-...sd-m2-1022-mac
One reason I went for it was I saw a youtube video of a guy with my exact laptop, and the exact SSD I purchased, and he linked the exact adapter I needed which I ordered, plus the SSD was on a decent sale so I could probably get rid of it if I had to for no loss. Otherwise I might not have tried this whole DIY it way.