A-Tech 16GB (2x8GB) RAM for Apple iMac 2017 (27 inch Retina 5K & 21.5 inch Retina 4K) | DDR4 2400MHz SODIMM PC4-19200 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at 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 : Amazon.ca: Electronics
How to upgrade iMac (2020) RAM and save up to $2000 - 9to5Mac
2017 21.5-inch 4K iMac memory upgrade kit includes 32GB of RAM, tools to take it apart | AppleInsider
The New 2017 iMac: Notes & Tidbits on RAM - The Mac Observer