Many technical reports are available online for free.
National Technical Reports Library (NTRL)
https://ntrl.ntis.gov/NTRL/
NTRL contains records for 3 million US federal technical reports dating back to 1964 and approximately 800,000 full-text reports, mainly from 1995 forward.
NSCEP/NEPIS - EPA's Gateway to Free Digital & Paper Publications
http://www.epa.gov/nscep/
NASA Technical Reports Server (NTRS)
https://ntrs.nasa.gov/search
Includes current and historical aerospace literature (technical reports, journal articles, conference papers, images, photos, etc.) back to 1915.
Science.Gov
http://www.science.gov/
The US government gateway to government sciencific information and research results.
Technical Report Archive and Image Library (TRAIL)
http://www.technicalreports.org/trail/search/
This site provides access to US federal reports up to 1975.
Virtual Technical Reports Center / University of Maryland Libraries
https://lib.guides.umd.edu/techreports
A browseable site by institutions for technical reports, e-prints, reprints, dissertations, theses and research reports.