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
Install memory in an iMac - Apple Support
NEMIX RAM Compatible with 256GB 4x64GB iMac Pro 27" 2017 Memory DDR4-2666 PC4-21300 RDIMM at Amazon.com