Logic Board 3.3 GHz i7 16GB Ram iMac Retina 4K 21.5" Late 2015 A1418 W - Part Haven
New 21.5-inch iMac least repairable yet due to lack of RAM, hard drive upgradability - 9to5Mac
2015 - 27" Retina iMac, 3.2GHz Quad Core i5 Processor, 16GB RAM, 1TB F – The Apple Xchange - Preowned Apple Products and Services
27 inch iMac with Retina 5k Display 2019 3.7GHz i5 1TB SSD 64GB RAM | Refurbished | Techable
Install memory in an iMac - Apple Support
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
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