Policy papers are available from a wide variety of think tanks and interest groups. Some are truly non-partisan while others have definite political perspectives. For example: the American Enterprise Institute (conservative) and the Brookings Institution (Liberal) will have very different perspectives on the same issue. Keep these political perspectives (agenda) in mind as you use policy papers and try to find papers that present differing perspectives--and make your own judgement!.
Working papers are informational reports, produced by an agency as part of the ongoing work of the organization. Or, they may be prepublication versions academic articles, book chapters, etc. Visit agency websites and look for their working papers. For international organizations use the search engines listed on the government information tab on the left.