8GB (2 x 4GB kit) OEM RAM Memory for Apple iMac Retina 5K 27 inch (Late 2014) Core i5 / i7 PC3-12800 DDR3-1600MHz 204-Pin SoDimm at Amazon.com
iMac with Retina 5K Display 2014/2015 Memory Upgrades
Apple iMac (21.5-inch, 2014) review: A price cut for the 21-inch iMac makes it a mainstream machine - CNET
Install memory in an iMac - Apple Support
A-Tech 8GB RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com