Configure your own 21.5-inch Apple iMac (2013) at OWC
HOW I UPGRADED MY iMAC 21.5in Late 2013 MODEL (SSD & RAM UPGRADE) - YouTube
Memory upgrade: Apple iMac 2012/2013/2014 (Howto)
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
Ram For Imac 21.5 Late 2013 Hotsell, SAVE 33% - piv-phuket.com
Apple's new 'Assembled in USA' iMac a bear to upgrade, repair • The Register