Upgrading SSD on mid 2014 MB: which SSD?
Without having heard of this sort of incompatibility before, I'd suggest you check out Other World Computing, who have a great reputation for Mac aftermarket parts - including a great line of SSDs. I've been happy with one of their SSDs in an old MacBook Pro myself. Great product backed with a thorough understanding of Apple's product line and fantastic customer service. 

And to be clear, I'm not affiliated in any way other than being a happy customer. 

Update: Thanks to @enguerranws for doing the extra legwork; OWC indeed does have a solution available for the 11,1 in 240GB, 480GB and 1TB as of at least Dec 2016.