OFFTEK 2GB Replacement Memory RAM Upgrade for Apple iMac 2.5GHz Intel Quad-Core i5 - (21.5-Inch) (DDR3 - Mid 2011) (DDR3-10600) Desktop Memory at Amazon.com
How to install RAM in an iMac (video) - CNET
4GB (2X2GB) DDR2 MEMORY FOR Apple iMac 2.8GHz Core 2 Duo 24" (MB325LL/A) - Walmart.com
Kingston Technology 4 GB Memory for Select Apple Imac's and Macbooks Single (Not a kit) 1066 MHz (PC3 8500) 204-Pin DDR3 SO-DIMM KTA- MB1066/4G at Amazon.com
OWC 8GB (2 x 4GB) PC12800 DDR3L 1600MHz SO-DIMMs Memory Compatible with 2011-2015 iMac, 2011-12 Mac Mini, and 2011-2012 MacBook Pro (Non-Retina Display) Models (OWC1600DDR3S08S) at Amazon.com
Memory RAM Upgrade for Apple iMac 21.5-inch Mid 2011 4GB/8GB DDR3 SODIMM | eBay
1600 MHz RAM in iMac 2011 ? | MacRumors Forums
Apple iMac Mid 2011 21.5-inch 2.7GHz Core i5 Apple DDR3 RAM Memory | Kingston
NeweggBusiness - Kingston 4GB (2 x 2GB) DDR2 800 (PC2 6400) Dual Channel Kit Memory for Apple iMac Model KTA-MB800K2/4G
Teardown of new 21.5-inch iMac surprisingly reveals user-upgradable RAM, modular CPU - 9to5Mac
Amazon.com: Kingston KCP316SD8/8 8gb 1600mhz Sodimm Mem : Home & Kitchen