How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
A-Tech 32GB Kit (4x8GB) RAM for Apple iMac (Late 2012, Late 2013, Late 2014, Mid 2015) | DDR3 1600MHz SODIMM PC3-12800 204-Pin SO-DIMM Max Memory Upgrade at Amazon.com
Apple iMac A1311 21.5" Desktop - MC509LL/A (July, 2010) for sale online | eBay