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
Apple iMac 21.5" Mid 2011 Intel Core i5 2.7 GHz 1TB SATA Disk 4GB RAM HighSierra | eBay
Apple iMac 21.5-inch | Extended Warranties & Free Shipping
Apple iMac 2011 Memory 4GB DDR3 1333MHz PC3-10600 204-Pin
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 MC309LL/A 21.5 inch iMac Core i5 2.5 GHz 4GB RAM 500GB HDD Mid-2011 | Recycledgoods.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
Apple iMac 21.5" All-in-One Desktop Computer i3 3.07GHz 4GB Ram 500GB Mac OS High Sierra w/ Keyboard & Mouse - GRADE B
4GB PC10600 DDR3 1333MHz SO-DIMM Memory Compatible with 2011 MacBook Pro, Mid 2010/2011 21.5" & 27" iMac Models, 2011 Mac Mini (OWC1333DDR3S4GB) at Amazon.com
Apple MC309LL/A 21.5 inch iMac Core i5 2.5 GHz 4GB RAM 500GB HDD Mid-2011 | Recycledgoods.com