A-Tech 32GB (4x8GB) RAM for Apple iMac Late 2015 27 inch Retina 5K | DDR3L 1866MHz / 1867MHz PC3L-14900 1.35V 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Timetec 8GB Compatible for Apple Late 2015 iMac (27-inch w/Retina 5K Display) DDR3L 1867MHz / 1866MHz PC3L-14900 2Rx8 CL13 1.35V 204 Pin SODIMM Memory Module MAC RAM Upgrade for iMac 17,1 at Amazon.com
Timetec 32GB KIT(4x8GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM Upgrade at Amazon.com
Apple iMac 21.5" Retina 4K (Late 2015) 3.1GHz i5, 8GB RAM, 1TB HD Flawless | eBay
A-Tech 16GB (2x8GB) RAM for Apple iMac Late 2015 27 inch Retina 5K | DDR3L 1866MHz / 1867MHz PC3L-14900 1.35V 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Install memory in an iMac - Apple Support
iMac 27" 5K Late 2015 Upgrade 8Gb To 16Gb Ram in 4K - How to upgrade Ram of a iMac 5K - 2017 - YouTube