OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
Apple Updates iMac with Haswell and Faster PCIe Flash Storage - MacRumors
OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
How-To: Now's the right time to swap your old iMac's hard drive for a fast new SSD - 9to5Mac
Timetec 256GB MAC SSD NVMe PCIe Gen3x4 3D NAND TLC Read Up to 1,950MB/s Compatible with Apple MacBook Air (2013-2015, 2017), MacBook Pro (2013-2015), iMac (2013-2019), Mac Pro (2013), Mac Mini (2014) at Amazon.com
iMac late 2012 SSD Upgrade - Apple Community
2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica