Research and advocacy institutes are often known as think tanks. A think tank is an organization that conducts research and advocates in areas such as social policy. Think tank reports are another useful way to pursue research on policy issues.
Harvard Kennedy School provides a free Google Custom Search of more than 700 think tanks and research centers.
The University of Oregon's American Policy Search allows searches by topic.
Here are a few selected national think tanks/research institutes that work in areas relevant to social welfare. Please be aware that think tanks may be biased in one direction or another, so it's a good idea to investigate them and exert caution when using information from them in your research.