A-Tech 16GB (4 x 4GB) PC3-8500 DDR3 1066/1067 MHz RAM for iMac Late 2009 21.5-inch / 27-inch | 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Install memory in an iMac - Apple Support
OWC 6GB (2GB + 4GB) PC8500 DDR3 1066MHz SO-DIMMs Memory RAM Upgrade Compatible with Late 2008, Early 2009, Early 2010 MacBook, MacBook Pro Unibody Models, Mac Mini, 2009 iMac (OWC8566DDR3S6GP) at Amazon.com
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
Memory Upgrades for Apple iMac 2011 Up to 32GB
Early 2009 model iMac Computer, DDR3 8GB (2x4GB) RAM Replacement - YouTube