Timetec Hynix IC 32GB KIT(2x16GB) Compatible for Apple 2019 iMac 27-inch w/Retina 5K Display, Late 2018 Mac Mini DDR4 2666MHz PC4-21300 2Rx8 CL19 1.2V SODIMM Memory RAM Upgrade (32GB KIT(2x16GB)) - Newegg.com
Apple's New 21.5 Inch iMac Has Upgradeable Parts — That's a Huge Deal
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
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Amazon.com: A-Tech 8GB RAM for Apple iMac & Mac Mini (2018, 2019 & 2020) | DDR4 2666 MHz SODIMM PC4-21300 / PC4-21333 260-Pin SO-DIMM Memory Upgrade : Electronics
Amazon.com: A-Tech 32GB Kit (2x16GB) RAM for Apple iMac & Mac Mini (2018, 2019 & 2020) | DDR4 2666 MHz SODIMM PC4-21300 / PC4-21333 260-Pin SO-DIMM Memory Upgrade : Electronics
Install memory in an iMac - Apple Support
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iMac RAM Upgrades Up to 128GB - OWC
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
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