Amazon.com: OWC 8GB (2 x 4GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S08P) : Everything Else
Best Buy: Apple® 21.5" iMac® 4GB Memory 500GB Hard Drive MC309LL/A
Apple® iMac with Intel® Core i3 3.06GHz, 4GB RAM, 500GB HDD + Keyboard/Mouse - Pick Your Plum
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A-Tech 4GB 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
iMac Memory for Model 8.1 | DDR2-800MHz |4GB | MacMemory.com
Apple iMac 20" | All In One Computer | Intel Dual Core Processor | 4GB RAM | 320GB HDD | WIFI | Mac OSX