12 GB DDR4 2666 SDRAM | How to Get 12GB SDRAM on Dual-Channel - Hardware Centric
OWC 12GB (8GB+4GB) PC3-12800 DDR3L 1600MHz SO-DIMM 204 Pin CL11 Memory Upgrade Kit Compatible with iMac, Mac Mini, and MacBook Pro (OWC1600DDR3S12S) at Amazon.com
Crucial 12GB DDR3 SDRAM Memory Module (CT3KIT51272BB1339) at Amazon.com