16GB Kit (4 x 4GB) PC3-10600 RAM for Apple iMac Mid 2011 - 27 Inch MC814LL/A Memory Upgrade Kit
OWC 8GB (2 x 4GB) PC12800 DDR3L 1600MHz SO-DIMMs Memory Compatible with 2011-2015 iMac, 2011-12 Mac Mini, and 2011-2012 MacBook Pro (Non-Retina Display) Models (OWC1600DDR3S08S) at Amazon.com
Apple iMac 21.5-inch | Extended Warranties & Free Shipping
Basic Apple iMac with High Sierra OS | i-series processor | 4 Gb memory — Free Geek Twin Cities
Crucial 4GB DDR3L-1333 SODIMM Memory for Mac | CT4G3S1339M | Crucial.com
A-Tech 4GB RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010, Mid 2011, Late 2011), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Module at Amazon.com