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
Apple iMac A1312 27 inch (1TB, Intel Core i5 2nd Gen., 2.7GHz, 4GB) Desktop - Silver - MC813LL/A for sale online | eBay
Apple iMac 27" i5 2.7 (Mid 2011) - Technical data and benchmarks
iMac Intel 21,5" Mid 2011 EMC 2428 - used original spareparts for Macs
Apple 27" iMac 16GB 1TB | Laptop Workshop
Install memory in an iMac - Apple Support
iMac 21,5" Mi 2011 - Intel i5 2,5 GHz 8 Go RAM Reconditionné | Occasion
Install memory in an iMac - Apple Support
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