How to upgrade the 27-inch 5K iMac (2019) with a massive 128GB of RAM [Video] - 9to5Mac
Amazon.com: OWC 128GB (2 x 64GB) PC23400 DDR4 ECC-R 2933MHz RDIMMs Memory Compatible with Mac Pro 2019 and up Models (OWC2933R6M128) : Everything Else
OWC 16.0GB (2 x 8GB) 2666MHz DDR4 PC4-21300 SO-DIMM 260 Pin Memory Upgrade, (OWC2666DDR4S16P), for 2018 Mac Mini (macmini18,1), 2019 27 inch iMac (iMac19,1) and PC laptops : Amazon.in: Computers & Accessories
Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
OWC 32.0GB 2666MHz DDR4 PC4-21300 SO-DIMM 260 Pin Memory Upgrade, (OWC2666DDR4S32G), for 2018 Mac Mini (macmini18,1), 2019 27 inch iMac (iMac19,1) and PC laptops … : Amazon.in: Computers & Accessories
OWC vs. Crucial memory for 27" Retina 5K iMac - 2019 i9, Model 19,1 | MacRumors Forums