Mac RAM Upgrade, Best iMac Memory Upgrade – Best Reviews
Configure your own 21.5-inch Apple iMac (2015) at OWC
A-Tech 32GB Kit (4x8GB) RAM for Apple iMac (Late 2012, Late 2013, Late 2014, Mid 2015) | DDR3 1600MHz SODIMM PC3-12800 204-Pin SO-DIMM Max Memory Upgrade at Amazon.com
Refurbished: Apple iMac 21.5-Inch "Core i5" 2.9GHz (Late 2012) A1418 MD094LL/A 8GB RAM 1TB HDD NVIDIA GT 650M MacOS MOJAVE USB Keyboard & Mouse Grade B - Newegg.com