OWC 64.0GB (4 x 16GB) PC3-14900 1866MHz DDR3 ECC-R SDRAM Memory Upgrade Kit, ECC Registered Compatible with Mac Pro 2013 (OWC1866D3R9M64)
Antonline
OWC 4 x 16.0GB PC3-14900 DDR3 Kit
Adorama
OWC / Other World Computing 64GB PC3-14900 1866MHz DDR3 Memory Upg. Kit, Mac Pro