8GB [2x4GB] DDR3-1333 RAM Memory Upgrade Kit for The Apple iMac 11,2 (21.5-inch, 3.2GHz, MC509LL/A) Intel Core i3 (Genuine A-Tech Brand) at Amazon.com
Install memory in an iMac - Apple Support
Amazon.com: Timetec 16GB KIT(2x8GB) Compatible for Apple DDR3 1333MHz PC3-10600 CL9 for Mac Book Pro (Early/Late 2011 13/15/17 inch), iMac(Mid 2010, Mid/Late 2011 21.5/27 inch), Mac Mini(Mid 2011) MAC RAM Upgrade :
Timetec Hynix IC 16GB KIT(4x4GB) Compatible for Apple Late 2009 iMac 21.5-inch / 27-inch DDR3 1067MHz/1066MHz PC3-8500 CL7 204 Pin 1.5V SODIMM RAM Upgrade for iMac 10,1 iMac 11,1 (16GB KIT(4x4GB)) - Walmart.com