How to Install Memory iMac 27-inch 5K (RAM Crucial 16GB) - YouTube
Timetec 64GB KIT (4x16GB) 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
Memory Upgrades for 2017 iMac 27-Inch
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