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
OWC Confirms New Low-End 27-inch iMac Can Be Fitted With 64GB RAM - MacRumors
Apple iMac (27-inch, 2017) review: Apple doesn't mess with success - CNET
Adamanta 16GB (2x8GB) Memory Upgrade Compatible for 2017 Apple iMac 27"/21.5" Retina 5K/4K or Non-Retina Display DDR4 2400MHz PC4-19200 SODIMM 1Rx8 CL17 1.2v Single Rank RAM DRAM at Amazon.com
A-Tech 16GB (2x8GB) RAM for Apple iMac 2017 (27 inch Retina 5K & 21.5 inch Retina 4K) | DDR4 2400MHz SODIMM PC4-19200 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at Amazon.com
iMac 27 inch Retina 5k i7 4.2GHz 16GB RAM 256/512/1T SSD Radeon Pro 575 4GB 2017 | eBay
Refurbished 27-inch iMac Pro 3.2GHz 8-core Intel Xeon W with Retina 5K display - Apple