NEMIX RAM 16GB Kit (2x8GB Upgrade) DDR3-1600 SODIMM PC3-12800 for Apple MacBook Pro Mid 2012 / iMac Late 2012 / iMac 2013, Late 2014, Mid 2015, Mac Mini Late 2012 at Amazon.com
Configure your own 21.5-inch Apple iMac (2013) at OWC
WiFi, IO & The Chassis - 21.5-inch iMac (Late 2013) Review: Iris Pro Driving an Accurate Display
Install memory in an iMac - Apple Support
Apple Posts iMac (Late 2013) Memory Specifications
Restored Apple iMac 21.5-inch 2.9GHZ Quad Core i5 (Late 2013) ME087LL/A 8 GB DDR3 1 TB HDD 1920 x 1080 Display Sierra 10.12 Includes Keyboard and Mouse (Refurbished) - Walmart.com
Memory Upgrades for iMac 21.5-Inch 2012 - Late 2013
How to Upgrade/Install Memory in a 21.5-inch iMac (Late 2012– Mid 2014) iMac14,1 iMac14,3 - YouTube
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