maxupgrades.com: Single SSD Drives kit for iMac 27 inch 2012 Models with SATA Blade SSD Module / Heatsink/ Power Cable and DATA Cables for [SSD + HDD] with iMac Adhesive LCD Tape
Amazon.com: SATA Blade SSD Module - Dual SSD Drive Kit for iMac 27 Inch 2012 Model : Electronics
You won't be adding an aftermarket SSD to your new iMac | Ars Technica
Amazon.com: OWC 1TB Aura Pro 6G 3D NAND Flash SSD Compatible with iMac (Late 2012) : Electronics
iMac late 2012 SSD Upgrade - Apple Community
M.2 NGFF B Key SSD to Compatible for iMAC A1418 2012 A1419 EMC:2544 Adapter
Internal SSD Install Not Recognized on iMac 2012 | MacRumors Forums