I just completed this upgrade using a regular SSD and an adaptor for my mid 2012 retina macbook pro.
You have two options:
1. Buy an m2 form factor SSD from OWC or from ifixit
2. Buy a regular ssd and buy a msata to M.2 adaptor - here are the ones I used:
* SAMSUNG 850 EVO mSATA 1TB Mini-SATA (mSATA) Internal SSD Single Unit Version MZ-M5E1T0BW
* mSATA SSD Adapter for MacBook Pro Retina A1425 MD212 MD213 ME662 (52 Pin)
* mSATA to USB 3 adaptor and M.2 to USB3 adaptor
Then
1. Backup your data to an external drive or two
2. Confirm your backup works
3. Put the mSATA SSD into the USB 3 adaptor
4. Clone the internal SSD into the mSata SSD (use internet booting on your MBP or the backup from step #1
5. Follow the iFixit tear down guide to extract the SSD and install the mSATA SSD (that you have put into the MSATA SSD to M.2 Adaptor