How to Upgrade Memory in a 21.5-inch or 27-inch iMac (Mid 2010) - YouTube
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
32.0GB OWC Memory Upgrade Kit For 2010-2011 Apple iMac
How to upgrade RAM on iMac 21.5" 2011 - YouTube
16GB -2x8GB Memory Ram Upgrade for 27" Apple iMac-12.2 Core i5 3.1GHz Mid 2011 | eBay
Amazon.com: Timetec 16GB KIT(2x8GB) Compatible for Apple DDR3 1333MHz PC3-10600 CL9 for Mac Book Pro (Early/Late 2011 13/15/17 inch), iMac(Mid 2010, Mid/Late 2011 21.5/27 inch), Mac Mini(Mid 2011) MAC RAM Upgrade :
Install memory in an iMac - Apple Support
Apple iMac 2.5Ghz i5-2400S (Mid-2011) *No HDD/RAM* – MSU Surplus Store
APPLE IMAC 2011 , ALL-IN-ONE , 22″ inch , Intel Core i5 -2500S - 2.7 G – Montag Store
Apple iMac MC309LL/A Mid 2011 Silver 21.5-inch- 8GB RAM - 500GB HDD - 2.5GHz i5-2400S (Scratch and Dent) - Walmart.com
Apple iMac 2.5Ghz i5-2400S (Mid-2011) *No HDD/RAM* – MSU Surplus Store