OWC 128GB (4 x 32GB) PC21300 DDR4 2666MHz SO-DIMMs Memory Compatible with 27" iMac w/Retina 5K (Early 2019 and up) and Compatible PCs (OWC2666DR4S128S) at Amazon.com
Samsung M471A1K43BB0-CPB 8GB PC4-17000 DDR4-2133MHz Non-ECC Unbuffered CL15 260-Pin SoDimm 1.2V Single Rank Memory Module - OEM at Amazon.com
Amazon.com: OWC 32GB PC19200 DDR4 2400MHz SO-DIMM Memory Compatible with 21.5" and 27" iMac Retina Models (Mid 2017), Mac Mini (Late 2018), and Compatible PCs (OWC2400DDR4S32G) : Everything Else
v-Color 32GB (2 x 16GB) ECC SO-DIMM DDR4 2666MHz (PC4-21300) for NAS Micro Server/Workstation/Compatible with Synology 21 Series 2Rx8 CL19 1.2V (TES416G26D819K-VKC) at Amazon.com