Amazon.com: Late 2015 Apple iMac with 3.2GHz Intel Core i5 (27 inch Retina 5K, 8GB RAM, 256GB SSD) Silver (Renewed) : Electronics
Apple 27" iMac Desktop | Intel Core i5-10600 - 8GB RAM - AMD Radeon Pro 5300 Graphics - 512 GB SSD in Silver with 3-Year AppleCare+ | NFM
OWC 8GB (2 x 4GB) PC12800 DDR3L 1600MHz SO-DIMMs Memory Compatible with 2011-2015 iMac, 2011-12 Mac Mini, and 2011-2012 MacBook Pro (Non-Retina Display) Models (OWC1600DDR3S08S) at Amazon.com
Install memory in an iMac - Apple Support
5K iMac (Late 2015) RAM Upgrade - YouTube
How to Install Memory Into a 27-Inch Apple iMac (Late 2015 - 2020) iMac17,1 iMac18,3 iMac19,1 - YouTube
OWC 32GB (4 x 8GB) PC14900 DDR3 1866MHz SO-DIMMs Memory Compatible with 2015 (Late) iMac 27" w/Retina 5K Models and Compatible PCs (OWC1867DDR3S32S) at Amazon.com
Memory RAM Upgrade for Apple iMac 27-inch Late 2013 4GB/8GB DDR3 SODIMM | eBay