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
Teardown of new 21.5-inch iMac surprisingly reveals user-upgradable RAM, modular CPU - 9to5Mac
Memory Upgrades for 2017 iMac 27-Inch
Install memory in an iMac - Apple Support
Apple iMac 21.5" - QUAD CORE - 8GB RAM - 2TB - THUNDERBOLT - macOS High Sierra | eBay
Install memory in an iMac - Apple Support
Apple iMac 21.5" Retina 4K - Mid 2017 Reviews, Pros and Cons | TechSpot