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
2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica
Sintech M.2 nVME SSD Adapter Card Upgrade Kits,Compatible for MacBook Air( 2013-2017 Year) and MacBook PRO(Late 2013-2015 Year,iMac
iMac 2013 (and later) with NVME SSD? | MacRumors Forums