DoDI 1015.15 - Listing of Accepted Users
Listing of Accepted Issuers
According to DoDI 1015.15, NAF investment are ONLY "authorized for obligations of the Federal Government, or for organizations or institutions sponsored or guaranteed by the U.S. Government."  These investment vehicles include issuances by the U.S. Treasury and "obligations of Government agencies, guaranteed as to principal and interest by the U.S. Government and obligations issued by U.S. Government-sponsored enterprises"  
 
Examples of authorized Federal agency and Government-sponsored enterprise issuers are included in the PDF document here:
DODI 1015.15 Listing of Accepted Issuers