OWC 16GB (4 x 4GB) PC8500 DDR3 1066MHz SO-DIMMs Memory Compatible with iMac 21.5" and 27" (October/2009) at Amazon.com
Memory upgrade compatibility list for iMac 2019-2020 | MacRumors Forums
Install memory in an iMac - Apple Support (HK)
Memory Upgrades for iMac 27-Inch (2012 - Mid 2015)
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
How to upgrade the 27-inch 5K iMac (2019) with a massive 128GB of RAM [Video] - 9to5Mac
Timetec Hynix IC 32GB KIT(4x8GB) Compatible for Apple Late 2009 iMac 27inch DDR3 PC3-8500 1067MHz/1066MHz CL7 204 Pin 1.5V Dual Rank 2R8 Memory Module RAM Upgrade for iMac 11,1 (32GB KIT(4x8GB)) -