PolicyArchive, CIAO, and PAIS are good starting points in the search for policy papers. Check out the other sources below, or the tabs for Think Tanks and NGOs/IGOs, for additional resources.
Many fantastic public policy resources--in particular, policy briefs from think tanks, NGOs, and research institutes--can be found online. Here are a few tips for more efficient, effective Google searches:
Include the phrase "policy brief" in your search. (Keep the quotation marks around "policy brief" to find this exact phrase.)
Use a site: search. This will limit your search to a broad domain (e.g., .org sites, .edu sites, .gov sites) or to a specific website (e.g., http://www.urban.org).
If you're not sure which think tank, NGO, or agency might have issued policy papers on your topic, try a search engine that's powered by Google but limited to a select group of sites.