How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
16GB 2X 8GB DDR3 RAM MEMORY FOR APPLE IMAC MODEL# IMAC11,2 IMAC11,3 A1311 A1312 | eBay
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