Technical reports may be difficult to identify because there is no central coordination for dissemination and publication. The report series code—a series of letters and numbers assigned by the issuing agency—can be useful for identifying and finding technical reports.
Science.gov
Includes 200 million pages of science information and R&D results for 36+ U.S. government agencies
List of specialized U.S. government databases available for searching
Search engine that works across all of the above U.S. gov't databases at the same time
TRAIL (Technical Report Archive and Image Library)
The Technical Report Archive & Image Library (TRAIL) contains digitized U.S. government agency technical reports, primarily issued prior to 1976. Material is from a number of current and former United States government agencies, including report series from the Atomic Energy Commission, Bureau of Mines, National Bureau of Standards, and National Advisory Committee for Aeronautics. See the full listing on TRAIL.
WorldWideScience.org
Also includes the information within Science.gov (listed above) and is a gateway to national and international scientific databases. You can search resources from 17 countries.