Indexes "gray literature"--technical reports, working papers, government and agency reports, conference proceedings, etc.-- available online or in the Rutgers Don M. Gottfredson Library of Criminal Justice.
CRS Reports
The Congressional Research Service (CRS) serves as shared staff to congressional committees and Members of Congress. CRS analyzes and reports on current and proposed policies.
Think Tank directory from the Harvard Kennedy School of Government. Searches across over 1200 think tanks and research centers. Focuses on websites of institutions that generate public policy research, analysis, and activity.
When reading reports from research agencies with which you are not familiar, be sure and check their "About" page (usually at the top or bottom of their web site.) It should give you a sense of their mission and possible biases in their reporting.