iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
Apple iMac A1311
iMac A1311 Ram Upgrade - Bing - Shopping
Timetec 32GB KIT(4x8GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM Upgrade at Amazon.com
iMac Repair & Upgrade, Same Day Services
A-Tech 32GB (4x8GB) RAM for Apple iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Apple iMac 21.5" A1311 Intel Core i3 3.06GHz 4GB 250 HD SIERRA 2010 GRADE B | eBay
Memory upgrade: Apple iMac 2012/2013/2014 (Howto)
New 16GB 4x4GB Memory Apple iMac (21.5 and 27-inch, Mid 2011) PC3-10600 at Amazon.com