How to Upgrade Memory in a Late 2009 21.5-inch or 27-inch iMac - YouTube
Install memory in an iMac - Apple Support
iMac Memory for Models 10.1 and 11.1 | 16GB (4GBx4) | MacMemory.com
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
iMac Pro Memory Maxxer RAM Upgrade Kit
Timetec 16GB KIT(4x4GB) Compatible for Apple DDR3 1067MHz / 1066MHz PC3-8500 CL7 Dual Rank for iMac (Late 2009 21.5-inch / 27-inch) SODIMM Memory MAC RAM Upgrade for iMac 10,1 and iMac 11,1 at Amazon.com