Amazon.com: 2017 Apple iMac with Intel Core i5 (21.5-inch, 8GB RAM, 1TB Storage) - Silver (Renewed) : Electronics
8GB (2 x 4GB kit) OEM RAM Memory for Apple iMac Retina 5K 27 inch (Late 2014) Core i5 / i7 PC3-12800 DDR3-1600MHz 204-Pin SoDimm at Amazon.com
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 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors
Upgrade your iMac 27'' 2017 RAM with MrMemory review - Wim Arys
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac