The New 2017 iMac: Notes & Tidbits on RAM - The Mac Observer
Install memory in an iMac - Apple Support
iMac Intel 21.5" Retina 4K Display (2017) RAM and SATA Hard Drive Upgrade - iFixit Repair Guide
Amazon.com: 2017 Apple iMac with Intel Core i5 (21.5-inch, 8GB RAM, 1TB Storage) - Silver (Renewed) : Electronics
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