A-Tech 8GB 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 Module at Amazon.com
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
Configure your own 27-inch Apple iMac (2017) at OWC
Apple iMac 27 Inch 5K 3.5 GHz Core i5 1TB Fusion 64GB RAM 4GB GFX 2017 – Quality Macs
2017 iMac
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
Apple iMac 27-Inch with 5K Display Review (2017): Serious Power | Tom's Guide