Amazon.com: OWC 64GB (4 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with 27" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S64S) : Everything Else
Apple Memory Module 32GB DDR4 2400MHz SO-DIMM (2x16GB) - Apple
Apple Memory Module 16GB DDR4 2400MHz SO-DIMM (2x8GB) - Apple
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
Crucial 16GB DDR4-2400 SODIMM Memory for Mac | CT16G4S24AM | Crucial.com
8GB Crucial DDR4 SO-DIMM 2400MHz CL17 Memory Module Apple iMac with Retina 5K 649528783295 | eBay
iMac 2019 - upgraded w 2 extra ram and the speed dropped to 2400 MHz? | MacRumors Forums
Timetec 16GB Compatible for Apple 2017 iMac (27-inch w/Retina 5K, 21.5-inch w/Retina
Apple Memory Module 16GB DDR4 2400MHz SO-DIMM (2x8GB) - Apple
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