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
Install memory in an iMac - Apple Support
Refurbished: Apple iMac 21.5-Inch "Core i5" 2.3GHz (Mid-2017) MMQA2LL/A A1418 8GB RAM 1TB HDD MacOS Mojave Apple USB Keyboard & Mouse - Newegg.com