Best Buy: Apple® 21.5" iMac® 4GB Memory 500GB Hard Drive MC309LL/A
21.5" iMac (Mid 2011) Model ID:iMac12,1 - TransIntl
Install memory in an iMac - Apple Support
Timetec Hynix IC 32GB KIT(4x8GB) Compatible for Apple 27 inch Mid 2010 21.5/27 inch Mid 2011 iMac DDR3 1333MHz PC3-10600 CL9 204 Pin SODIMM Upgrade for iMac 11,3 iMac 12,1 iMac 12, 2 (32GB KIT(4x8GB)) - Newegg.com
Differences Between Mid-2011 and Late 2012 Aluminum iMac: EveryMac.com
Best Buy: Apple® iMac® 21.5" 8GB Memory 1TB Hard Drive A1311
Apple iMac 2011 (Mid) 27-inch 2K (iMac 12,2) SSD ,Ram & OSX Upgrades - ZD Laptop Service Centre
Memory Upgrades for Apple iMac 2011 Up to 32GB
Upgrade your iMac's RAM in less than 5 minutes - YouTube
Apple iMac A1311 21.5" AiO Computer i5-2400S (Mid-2011) -
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