A-Tech 64GB (4x16GB) RAM for Apple iMac 2017 27 inch Retina 5K | DDR4 2400MHz SODIMM PC4-19200 2Rx8 1.2V 260-Pin SO-DIMM Memory Upgrade Kit
Timetec 16GB Compatible for Apple 2017 iMac (27-inch w/Retina 5K, 21.5-inch w/Retina 4K / Non-Retina 4K) DDR4 2400MHz PC4-19200 CL17 SODIMM Memory MAC RAM Upgrade for iMac 18,1 / iMac 18,2 /
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
Install memory in an iMac - Apple Support
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
YouTuber upgrades base-model 5K iMac with seventh-gen Intel Core i7 processor, 64 GB of RAM, and more [Video] - 9to5Mac
NEMIX RAM Compatible with 256GB 4x64GB iMac Pro 27" 2017 Memory DDR4-2666 PC4-21300 RDIMM at Amazon.com