OWC 16GB (4 x 4GB) PC8500 DDR3 1066MHz SO-DIMMs Memory Compatible with iMac 21.5" and 27" (October/2009) at Amazon.com
A-Tech 16GB (4 x 4GB) PC3-8500 DDR3 1066/1067 MHz RAM for iMac Late 2009 21.5-inch / 27-inch | 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Install memory in an iMac - Apple Support
Timetec 32GB KIT(4x8GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM Upgrade at Amazon.com