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 64GB of RAM in Apple 27" iMac 2017
A-Tech 64GB (4x16GB) RAM for Apple iMac 2017 27 inch Retina 5K | DDR4 2400MHz SODIMM PC4-19200 2Rx8 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at Amazon.com
iMac Intel 21.5" Retina 4K Display (2017) RAM and SATA Hard Drive Upgrade - iFixit Repair Guide
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
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Memory Upgrades for 2017 iMac 27-Inch
How To: Upgrade 2017 5k iMac RAM to 64GB & Saved $840 | 2018 - YouTube
Install memory in an iMac - Apple Support
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Amazon.com: 2017 Apple iMac with Intel Core i5 (21.5-inch, 8GB RAM, 1TB Storage) - Silver (Renewed) : Electronics
Install memory in an iMac - Apple Support
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac