How to Speed up your iMac with a NVMe 960EVO M.2 SSD - YouTube
How to Upgrade/Replace the PCIe SSD in a 21.5-in iMac (2019)
256GB PCIe Blade SSD Kit - Apple iMac 21.5" A1418 Late 2013,Mid 2014,2015 4K | eBay
For iMac Mac mini Macbook Pro and Retina Mac Pro SSD FASTEST PCIe 3.0 x4 POLARIS NVMe FLASH SSD 32GB 128GB 256GB 512GB 1TB 2TB - AliExpress Computer & Office
How to Install/Upgrade the PCIe SSD in a 21.5-inch iMac (Mid 2017) iMac18,1 iMac18,2 - YouTube
DATARAM 256GB M.2 M-Key PCIe NVMe SSD for 2013-16 MacBook, Mac Pro, Air, Mini, iMac - Newegg.com
Timetec 512GB MAC SSD NVMe PCIe Gen3x4 3D NAND TLC Read Up to 2,000MB/s Compatible with Apple MacBook Air (2013-2015, 2017), MacBook Pro (2013-2015), iMac (2013-2019), Mac Pro (2013), Mac Mini (2014) : Electronics
2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica