A-Tech 16GB (4x4GB) RAM for Apple iMac (Mid 2010, Mid 2011, Late 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Apple 21.5" iMac® Intel Core i5 (2.3GHz) 8GB Memory 1TB Hard Drive Silver MMQA2LL/A - Best Buy
HOW TO: Upgrade The RAM to 32GB on a Mid-2011 21.5inch iMac - YouTube
Install memory in an iMac - Apple Support
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