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
Install memory in an iMac - Apple Support
Teardown of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors
Apple's new 21.5" iMac with 4K Retina screen has user replaceable RAM and modular CPU
Install memory in an iMac - Apple Support
Apple's New 21.5 Inch iMac Has Upgradeable Parts — That's a Huge Deal
Refurbished Apple iMac 18,2/i5-7500/8GB RAM/1TB HDD/21.5-inch 4K RD/AMD Pro 560+4GB/C (Mid - 2017) | Mac4sale
NEMIX RAM 64GB (4X16GB) SODIMM Memory Upgrade Kit DDR4 2400MHz PC4-19200 Compatible with Apple iMac
Install memory in an iMac - Apple Support
Mac RAM Upgrade, Best iMac Memory Upgrade – Best Reviews