Amazon.com: OWC 64.0GB (4 x 16GB) PC3-14900 1866MHz DDR3 ECC-R SDRAM Memory Upgrade Kit, ECC Registered Compatible with Mac Pro 2013 (OWC1866D3R9M64) : Everything Else
32GB (4x8GB) DDR3-1600MHz PC3-12800 ECC UDIMM 2Rx8 1.35V Unbuffered Server Memory by NEMIX RAM at Amazon.com
Crucial Mac Pro Systems (Late 2013) 32GB (2 x 16GB) 240-Pin DDR3 SDRAM DDR3 1866 (PC3 14900) Memory For Mac Pro Systems Model CT2K16G3R186DM - Newegg.com
OWC 4.0GB PC14900 DDR3 1866MHz ECC Memory Module at MacSales.com