How to Upgrade Memory in a Late 2009 21.5-inch or 27-inch iMac - YouTube
A-Tech for Apple 1GB DDR2 667MHz PC2-5300 200-pin SODIMM for MacBook, MacBook Pro, iMac, Mac Mini - (Early/Mid/Late 2006, Mid/Late 2007, Early 2008, Early 2009) - Memory RAM Module at Amazon.com
iMac RAM for Model 11.2 to 12.2 | DDR3-1066MHz | 8GBx2 | MacMemory.com
A-Tech 4GB PC3-8500 DDR3 1066/1067 MHz RAM for MacBook, MacBook Pro, iMac, Mac Mini (Late 2008, Early/Mid/Late 2009, Mid 2010) | 204-Pin SODIMM Memory Upgrade Module at Amazon.com
A-Tech for Apple 8GB Kit (2X 4GB) DDR3 1067MHz / 1066MHz PC3-8500 SODIMM MacBook, MacBook Pro, iMac, Mac Mini (Late 2008 Early/Mid/Late 2009 Mid 2010) Memory RAM at Amazon.com