NEMIX RAM 16GB (2x8GB Upgrade Kit) DDR3-1866 SODIMM Memory Compatible with Apple iMac 27" Late 2015 with Retina 5K Display 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
Third-Party RAM for 27-inch iMac Still Far More Affordable Than Apple's Checkout Upgrade Options - MacRumors
Don't Upgrade The RAM In Your New Apple iMac, Do This Instead