iMac RAM for Model 11.2 to 12.2 | DDR3-1066MHz | 4GBx2 | MacMemory.com
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
Install memory in an iMac - Apple Support
OWC1600DDR3S16P OWC 16GB (2x8GB) PC3-12800 DDR3L 1600MHz SO-DIMM 204 Pin CL11 Memory Upgrade Kit Compatible with iMac, Mac Mini, and MacBook Pro
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
32.0GB OWC Memory Upgrade Kit For 2010-2011 Apple iMac
A-Tech 32GB (4x8GB) RAM for Apple iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
CMS 16GB (4X4GB) DDR3 10600 1333MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with Apple® iMac Core I5 2.5 21.5-Inch (Mid-2011) - A28 at Amazon.com
NEMIX RAM 32GB (4x8GB Upgrade) DDR3-1333 PC3-10600 204-Pin SODIMM Memory for Apple MacBook Pro Early/Late 2011, iMac Mid 2010 27 inch 4-Core, iMac Mid 2011 21.5/27” , Mac Mini Mid 2011 at Amazon.com
iMac 21.5" Mid 2011 How To Change/Upgrade Your Ram For Better Performance 2014 - YouTube