NEMIX RAM 64GB Memory Upgrade Kit (2x32GB) SODIMM DDR4 2666MHz PC4-21300 Compatible for Apple iMac Mid 2020 (iMac20,1 / 20,2) / Mid 2019 (iMac19,1) 27-inch w/Retina 5K Display, Late 2018 Mac Mini 8,1 at Amazon.com
Timetec 32GB KIT(2x16GB) Compatible for Apple DDR4 2666MHz / 2667MHz for Mid 2020 iMac (20,1/20,2) / Mid 2019 iMac (19,1) 27-inch w/Retina 5K, Late 2018 Mac Mini (8,1) PC4-21333 /PC4-21300 MAC RAM at Amazon.com
iMac Retina 5K: Memory upgrade (RAM)
How to Upgrade iMac RAM (20" and 24" Aluminum): EveryMac.com