iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
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
Install memory in an iMac - Apple Support
Memory Upgrades for Apple iMac 2011 Up to 32GB
How to Improve Multitasking with an iMac Ram Upgrade?
2GB DDR3 1333 SODIMM for Apple iMac (Mid 2010,2011, Late 2011), Mac Mi – Aplus Technology
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
iMac RAM for Model 11.2 to 12.2 | DDR3-1066MHz | 24GB | MacMemory.com
Maximum memory for 2011 iMac - Ask Different
install - Installing and Upgrading Ram on a mid 2011 iMac (different size RAM) - Ask Different
Timetec 16GB KIT(2x8GB) Compatible for Apple DDR3L 1600MHz for Mac Book Pro(Early/Late 2011,Mid 2012), iMac(Mid 2011,Late 2012,Early/Late 2013,Late 2014,Mid 2015), Mac Mini(Mid 2011,Late 2012) MAC RAM at Amazon.com
Adamanta 16GB (2x8GB) Memory Upgrade Compatible for Apple Mid 2010, Early 2011, Mid 2011, Late 2011 iMac, 2010, 2011 MacBook Pro, Mac Mini DDR3 1333Mhz PC3-10600 SODIMM 2Rx8 CL9 1.5v Module RAM at Amazon.com
New 16GB 4x4GB Memory Apple iMac (21.5 and 27-inch, Mid 2011) PC3-10600 at Amazon.com