OWC 6GB (2GB + 4GB) PC8500 DDR3 1066MHz SO-DIMMs Memory Compatible with Late 2008, Early 2009, Early 2010 MacBook, MacBook Pro Unibody Models, 2009/2010 Mac Mini, 2009 iMac Models at Amazon.com
Memory Upgrades for Apple iMac 2009 - Up to 32GB
iMac (2007-2009) Memory Installation Video - YouTube
Apple iMac Late 2009 All in One Pc i7 16gb Ram 2TB HHD 2.8gz Selling as Is | eBay
Catalina 10.15.3 running decent on my mid 2009 iMac. Gonna max out the RAM to 8GB. 🤗 : r/CatalinaPatcher
Install memory in an iMac - Apple Support
Apple iMac 21.5" All-in-One Desktop Computer i3 3.07GHz 4GB Ram 500GB HDD Mac OS High Sierra MC508LL/A w/ Keyboard & Mouse