Adamanta 16GB (2x8GB) Memory Upgrade Compatible for 2017 Apple iMac 27"/21.5" Retina 5K/4K or Non-Retina Display DDR4 2400MHz PC4-19200 SODIMM 1Rx8 CL17 1.2v Single Rank RAM DRAM at Amazon.com
Amazon.com: OWC 16GB (2 x 8GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with 27" iMac w/Retina 5K (Mid 2017), Mac Mini (Late 2018), and Compatible PCs (OWC2400DDR4S16P) : Everything Else
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
NEMIX RAM 64GB (4X16GB) SODIMM Memory Upgrade Kit DDR4 2400MHz PC4-19200 Compatible with Apple iMac