How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
How to Upgrade the RAM in Your iMac
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 :
iMac Intel 21.5" and 27" (Mid 2011) SSD Upgrade Bundle - iFixit Store
Adamanta 16GB (2x8GB) Apple Memory Upgrade DDR3L 1600MHz SODIMM Compatible for iMac (Mid 2011, Late 2012, Early/Late 2013, Late 2014, Mid 2015), MacBook Pro (Early/Late 2011, Mid 2012), Mac Mini at Amazon.com
Memory RAM Upgrade for Apple iMac 21.5-inch Mid 2011 4GB/8GB DDR3 SODIMM | eBay
A-Tech 16GB (2x8GB) 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 Kit at Amazon.com
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
Memory Upgrades for Apple iMac 2011 Up to 32GB
Install memory in an iMac - Apple Support
iMac 21.5" Mid 2011 How To Change/Upgrade Your Ram For Better Performance 2014 - YouTube
install - Installing and Upgrading Ram on a mid 2011 iMac (different size RAM) - Ask Different
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 :