upgrade - iMac 27" Late 2015 - what ram to buy? - Ask Different
OWC 8GB (2 x 4GB) PC12800 DDR3L 1600MHz SO-DIMMs Memory Compatible with 2011-2015 iMac, 2011-12 Mac Mini, and 2011-2012 MacBook Pro (Non-Retina Display) Models (OWC1600DDR3S08S) at Amazon.com
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET