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
Maximum memory for 2011 iMac - Ask Different
Apple iMac 21.5-inch Mid 2011 2.5GHz Intel Core i5 (MC309LL/A) – UNICOM, Inc.
Install memory in an iMac - Apple Support
Used Apple iMac 21.5" Mid 2011 i5-2400S 4GB DDR3 RAM 500GB HDD El Capitan Tested | eBay