Reports ("blue books") on audits, surveys, investigations, and evaluations of Federal programs conducted by the U.S. Government Accountability Office (GAO). GAO is an independent, nonpartisan agency that works for Congress. Often called the "congressional watchdog", GAO investigates how the federal government spends taxpayer dollars. Other GAO Reports can be found on the Government Printing Office website.
A comprehensive overview from Stanford University on crafting a white paper.
Finding white papers: White papers may be written by organizations seeking to influence policy or by those doing research for members of the legislature. Begin by identifying your target area for research, and then seek out organizations in that area. For example, the Pennsylvania State Education Association produces white papers on various topics of concern to educators and legislators in the state of Pennsylvania.