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 of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors
Amazon.com: Mid 2017 Apple iMac with 3.0GHz Quad Core i5 (21.5 inch Retina 4K, 16GB RAM 256GB SSD) (Renewed) : Electronics