The following website has listings of NSF certified sand suppliers and processing facilities: http://www.nsf.org/Certified/Common/Company.asp?Standard=061
Two of the companies listed at this nsf.org site have supplied the sand used in the filters described in the study described on this website They are:
MANUFACTURERS MINERAL COMPANY in Washington state and Cemex in Californa. They are wholesale. You will need to find a hardware store, or building supplier near you to order
products from them.