NEMIX RAM 32GB (2x16GB Upgrade Kit) DDR3-1866 SODIMM Memory Compatible with Apple iMac 27" Late 2015 with Retina 5K Display
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
Memory upgrade: Apple iMac 2012/2013/2014 (Howto)
Install memory in an iMac - Apple Support
OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com
Install memory in an iMac - Apple Support
Memory Upgrades for iMac w/ Retina 5K Display Late 2015