Amazon.com: OWC 8GB (2 x 4GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S08P) : Everything Else
A-Tech 64GB (2 x 32GB) RAM for Apple iMac Pro (2017) - DDR4 2666MHz PC4-21300 ECC RDIMM 288-pin Memory Upgrade Kit at Amazon.com
Timetec 32GB KIT (4x8GB) Compatible for Apple 2017 iMac (27-inch w/Retina 5K Display) DDR4 2400MHz PC4-19200 1Rx8 CL17 1.2V Non-ECC Non-Parity Unbuffered SODIMM Memory MAC RAM Upgrade for iMac 18,3 at Amazon.com
A-Tech 16GB (2x8GB) RAM for Apple iMac 2017 (27 inch Retina 5K & 21.5 inch Retina 4K) | DDR4 2400MHz SODIMM PC4-19200 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at Amazon.com
A-Tech 8GB RAM for Apple iMac 2017 (27 inch Retina 5K & 21.5 inch Retina 4K) | DDR4 2400MHz SODIMM PC4-19200 1.2V 260-Pin SO-DIMM Memory Upgrade Module at Amazon.com
Amazon.com: Apple iMac MNDY2LL/A 21.5 Inch, 3.0GHz Intel Core i5, 8GB RAM, 1TB HDD, Silver (Renewed), macOS High Sierra : Electronics