Apple 32GB (2 x 16GB) 2400MHz DDR4 SO-DIMM PC4-19200 Memory Upgrade for Late 2015-2017 iMac 27 inch with Retina 5K Display 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