Adamanta 16GB (1x16GB) Memory Upgrade Compatible for 2017 Apple iMac 21.5" Retina 4K Display and Non-Retina Display DDR4 2400Mhz PC4-19200 SODIMM 2Rx8 CL17 1.2v Dual Rank RAM DRAM at Amazon.com
Install memory in an iMac - Apple Support
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
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
Memory Upgrades for 2017 iMac 27-Inch
Teardown, testing discover Apple made 2017 iMacs easier for users to upgrade | AppleInsider