Think tanks are independent research organizations that have been created to conduct interdisciplinary research with the aim of providing expert advice on public policy issues. Although they are often described as non-partisan, this does not mean that they hold a neutral position on issues. The term "non-partisan" simply means that the organization does not hold the official endorsement of any political party.
Always vet information from think tanks thoroughly. Do outside research on the organization to determine where they get their funding and what individuals and organizations endorse and utilize certain think tanks. Compare a think tank's research to other research resources.
The Government Accountability Office (GAO) is an independent, nonpartisan agency that works for Congress. Often called the "congressional watchdog," GAO investigates how the federal government spends taxpayer dollars. While the focus of the reports are on fiscal analysis, they do provide excellent background information on policies that can be helpful to your overall policy analysis. Background information includes a review of literature, related laws, regulations, etc.