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
A-Tech 8GB RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Module at Amazon.com
Apple iMac 21.5" Mid 2011 A1311 i5 2.5GHz 12 GB Ram 2TB HD Radeon HD DVD GRADE A – I.E.T. INDUSTRIAL ANTONIO PRIETO – SINCELEJO, SUCRE
Install memory in an iMac - Apple Support
iMac 21.5" Mid 2011 How To Change/Upgrade Your Ram For Better Performance 2014 - YouTube