The New 2017 iMac: Notes & Tidbits on RAM - The Mac Observer
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
Amazon.com: OWC 32GB (4x8GB) 2400MHZ DDR4 SO-DIMM PC4-19200 Memory Upgrade Compatible with 2017 iMac 27 inch with Retina 5K Display (OWC2400DDR4S32S) : Everything Else
New iMac 21-5-inch 4K Model is More Upgradeable Than Past Machines | Digital Trends
Memory RAM Upgrade for Apple iMac 27-inch Mid 2017 8GB/16GB DDR4 SODIMM | eBay