Apple iMac (27-inch, 2017) review: Apple doesn't mess with success - CNET
Factory Original 64GB (2x32GB) Memory Upgrade for 2020 & 2019 Apple iMac 27" w/Retina 5K Display & 2018 Apple Mac Mini DDR4 2666Mhz PC4-21300 SODIMM 2Rx8 CL19 1.2v RAM Adamanta at Amazon.com
Apple 27" iMac® with Retina 5K display (Latest Model) Intel Core i7 (3.8GHz) 8GB Memory 512GB SSD Silver MXWV2LL/A - Best Buy
Apple iMac 27-inch (2020) review: new webcam, new screen option, same iMac - The Verge
Adamanta 16GB (1x16GB) Memory Upgrade for 2020 Apple iMac 27" w/Retina 5K Display, 2019 Apple iMac 27" w/Retina 5K Display, 2018 Apple Mac Mini DDR4 2666Mhz PC4-21300 SODIMM 2Rx8 CL19 1.2v DRAM
64GB (2x32GB) NEMIX RAM Memory for 2019 Apple iMac 27 inch Retina 5K (iMac19,1 A2115), 2018 Apple Mac Mini (Macmini8,1 A1993) - Newegg.com
Install memory in an iMac - Apple Support
Install memory in an iMac - Apple Support
Adamanta 16GB (1x16GB) Memory Upgrade for 2020 Apple iMac 27" w/Retina 5K Display, 2019 Apple iMac 27" w/Retina 5K Display, 2018 Apple Mac Mini DDR4 2666Mhz PC4-21300 SODIMM 2Rx8 CL19 1.2v DRAM
NEMIX RAM Compatible for 128GB (4x32GB) DDR4 2666MHz PC4-21300 SODIMM Memory Upgrade for Apple iMac Mid 2020 (iMac 20,1/20,2) / Mid 2019 (iMac19,1) 27-inch w/Retina 5K, Late 2018 Mac Mini 8,1 at Amazon.com
iMac Pro: Discontinued! Don't Buy an iMac Pro
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else