Apple iMac (27-inch, 2017) review: Apple doesn't mess with success - CNET
OWC 64GB DDR4 2400 MHz SO-DIMM Memory Upgrade Kit
OWC2400DDR4S16P Other World Computing DDR4, KIT, 16 GB: 2 X 8 GB, SO-DIMM 260-PIN, 2400 MHZ / PC4-19200, CL17, 1.2 V, UNBUFFERED, NON-ECC, UPGRADE, FOR APPLE IMAC (MID 2017) : PartsSource : PartsSource - Healthcare Products and Solutions
parts-quick 8GB (2 X 4GB) Kit Memory Module for Apple iMac 27" 2017 5K Retina Display DDR4 2400MHz SO-DIMM RAM at Amazon.com
Apple iMac (27-inch, 2017) review: Apple doesn't mess with success - CNET
iMac 2017, *must* the RAM be 2400MHz? | MacRumors Forums
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