iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Memory Upgrades for iMac 27-Inch (2012 - Mid 2015)
16GB (2x8GB) RAM Memory Upgrade For Apple iMac 27'' A1419 Late 2012 2013 | eBay
iMac Repair & Upgrade, Same Day Services
How to Install Memory Into a 27-Inch Apple iMac (Late 2015 - 2020) iMac17,1 iMac18,3 iMac19,1 - YouTube
How To Upgrade the RAM on the New 27-Inch iMac (2020) & Save Money! $$ - YouTube
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else
A-Tech 32GB Kit (4x8GB) RAM for Apple iMac (Late 2012, Late 2013, Late 2014, Mid 2015) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com