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
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
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
OFFTEK 2GB Replacement Memory RAM Upgrade for Apple iMac 2.5GHz Intel Quad-Core i5 - (21.5-Inch) (DDR3 - Mid 2011) (DDR3-10600) Desktop Memory at Amazon.com
How to Upgrade Memory in a 21.5-inch or 27-inch iMac (Mid 2010) - YouTube
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