While companies like Transcend do make upgrade kits for the older MacBook Pro retina series there are no upgrades for '14 model offered yet. Here's the link to Transcend Apple SSD Upgrades. I would check them out every other week or so to see when they release an option.
As to why you can't just swap out the SSD with just anything has a lot to do with the M.2 standard and how it does not define the I/O dialog. While you can find SSD's which look exactly the same as yours they will not function as the firmware on the SSD dictates the dialog.
Think of it this way we all can talk, but if someone spoke a different language to you wouldn't understand what was being said. Thats the difference here between the physical interface (M.2) and the I/O dialog (mSATA, PCIe or what ever Apple is doing in your model)."
Just a matter of time till crucial/transcend/owc etc get their act together and offer one....I'm waiting too but in the interim, I'm using a Transcend Jetdrive lite 128gb......fits in sd socket flush and although slow, ideal for those big files I need to take with me (oh and video/music etc.....)....pretty cheap too, £50 in UK for 128Gb
You can get a bigger SSD from Other World Computing at <http://www.macsales.com>. I've dealt with them several times, and they've been great each time.
