How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
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
16GB DDR4 2400MHz PC4-19200 SO-DIMM RAM Memory Upgrade for 2017 iMac 27 inch with 5K Retina Display (PARTS-QUICK) - Walmart.com
Memory RAM Upgrade for Apple iMac 27-inch Mid 2017 8GB/16GB DDR4 SODIMM | eBay