64GB RAM upgrade kit for 21.5-inch iMAC now offered by OWC - 9to5Mac
Timetec 16GB Compatible for Apple 2017 iMac (27-inch w/Retina 5K, 21.5-inch w/Retina
Apple's New 21.5 Inch iMac Has Upgradeable Parts — That's a Huge Deal
Install memory in an iMac - Apple Support
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