Used Apple iMac 27-inch (Retina 5K) 4.2GHZ Quad Core i7 (Mid 2017) MNED2LL/A-BTO - GainSaver
Install memory in an iMac - Apple Support
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
The New 2017 iMac: Notes & Tidbits on RAM - The Mac Observer
Memory Upgrades for 2017 iMac 27-Inch
OWC Confirms 64GB of RAM in Apple 27" iMac 2017
Upgrade your iMac 27'' 2017 RAM with MrMemory review - Wim Arys
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