32GB (4 x 8GB) PC10600 DDR3 1333MHz SO-DIMMs Memory Compatible with 2011 MacBook Pro, Mid 2010/2011 21.5" & 27" iMac Models, 2011 Mac Mini (OWC1333DDR3S32S) at Amazon.com
16GB Kit (4 x 4GB) PC3-10600 RAM for Apple iMac Mid 2011 - 27 Inch MC814LL/
16GB 4GB DDR3 SODIMM 1333MHz Memory Kit For Apple iMac 2011 12.1 12.2 Memory IT | eBay
Install memory in an iMac - Apple Support
Maximum memory for 2011 iMac - Ask Different
Timetec 16GB KIT(2x8GB) Compatible for Apple DDR3L 1600MHz for Mac Book Pro(Early/Late 2011,Mid 2012), iMac(Mid 2011,Late 2012,Early/Late 2013,Late 2014,Mid 2015), Mac Mini(Mid 2011,Late 2012) MAC RAM at Amazon.com