How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
A-Tech 8GB (2x4GB) 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
Amazon.com: OWC 16GB (2 x 8GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with 27" iMac w/Retina 5K (Mid 2017), Mac Mini (Late 2018), and Compatible PCs (OWC2400DDR4S16P) : Everything Else
Apple iMac 5K Retina 27" Desktop - 4.2GHz Quad-Core i7 - 16GB RAM - 2. – The Refurbished Apple Store
Install memory in an iMac - Apple Support
Amazon.com: 2017 Apple iMac with Retina 5K Display (27-inch, 8GB RAM, 512GB SSD Storage) (Renewed) : Electronics
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
Amazon.com: 2017 Apple iMac with Retina 5K Display (27-inch, 8GB RAM, 512GB SSD Storage) (Renewed) : Electronics
Apple iMac 5K Retina 27" Desktop - 4.2GHz Quad-Core i7 - 16GB RAM - 2. – The Refurbished Apple Store