Amazon.com: Timetec 8GB KIT(2x4GB) Compatible for Apple DDR3 1333MHz PC3-10600 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) RAM Upgrade : Electronics
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
RUSH SALE! Apple iMac 21.5 inch (16GB RAM upgraded) Mid 2011, Computers & Tech, Desktops on Carousell
Apple 21.5” iMac (Mid 2011); iMac12,1 Installation Guides and Specifications - Macfixit Australia