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
A-Tech 64GB (4x16GB) RAM for Apple iMac 2017 27 inch Retina 5K | DDR4 2400MHz
NEMIX RAM Compatible with 256GB 4x64GB iMac Pro 27" 2017 Memory DDR4-2666 PC4-21300 RDIMM at Amazon.com
Adamanta 16GB (1x16GB) Memory Upgrade for 2017 Apple iMac 27" Retina 5K Display DDR4 2400Mhz PC4-19200 SODIMM 2Rx8 CL17 1.2v Dual Rank RAM DRAM - Walmart.com
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
Apple iMac (27-inch, 2017) review: Apple doesn't mess with success - CNET