OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com
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
OWC has prepared RAM upgrades for the 27-inch 2019 iMac already - NotebookCheck.net News
OWC Memory
OWC Ram for Apple Mac Pro 6,1 Ram 128GB (4 x 32GB) PC3-10600 DDR3 ECC - Part Haven
Amazon.com: OWC 256GB (2 x 128GB) PC23400 DDR4 ECC-R 2933MHz LRDIMMs Memory Compatible with Mac Pro 2019 and up Models (OWC29D4LR0256) : Everything Else