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
Amazon.com: Mid 2017 Apple iMac with 3.0GHz Quad Core i5 (21.5 inch Retina 4K, 16GB RAM 256GB SSD) (Renewed) : Electronics
Teardown of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors
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
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 at Amazon.com
Install memory in an iMac - Apple Support
The New 2017 iMac: Notes & Tidbits on RAM - The Mac Observer
OWC Confirms 64GB of RAM in Apple 27" iMac 2017
iMac Intel 21.5" Retina 4K Display (2017) RAM and SATA Hard Drive Upgrade - iFixit Repair Guide