Pre-Owned Apple iMac 21.5-inch Desktop "Core i5" 2.9 (Late 2013) 8GB Memory 1TB HDD ME087LL/A - Best Buy
Apple iMac 21.5" All-in-One Desktop Computer Core i5-4570R 2.7GHz 8GB RAM 250GB HDD Mac OS Mojave Late-2013 w/ Keyboard & Mouse
Apple iMac 21.5-inch A1418 Late 2015 Logic Board i5 1.6GHz 8GB Memory non- Retina | eBay
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica
A-Tech 8GB Kit (2x4GB) RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com
Apple's new 'Assembled in USA' iMac a bear to upgrade, repair • The Register
Memory Upgrades for iMac 21.5-Inch 2012 - Late 2013
How to Upgrade/Install Memory in a 21.5-inch iMac (Late 2012– Mid 2014) iMac14,1 iMac14,3 - YouTube