On their website, the Department of State provides a list of think tanks that are involved in foreign policy research. I would recommend you start with this page to identify relevant think tanks if this will be relevant to your simulation and policy brief: Related Sites/Think Tanks on the Department of State website.
For more directories to think tanks, visit the Social Policy Research Guide.
Note: Some of the following are advocacy groups and will not always explicitly identify themselves as such. Cato, the American Enterprise Institute, and the Heritage Foundation run on the conservative side, for example; to counter, the Center for American Progress and the Economic Policy Institute are usually identified as liberal.
For a quick run-through of popular think tanks on the political spectrum, you can refer to this New York Time's Idea of the Day blog post that provides information on asset amounts, areas of research, political orientation, and their rating based on the University of Pennsylvania's Think Tank and Civil Societies Program (TTCSP).
Below is a list of some prominent think tanks involved in foreign policy research. For more, please visit the US Department of State's website.
According to the Lehman Social Sciences Library at Columbia University "the term "think tanks" is an imprecise phrase used to describe a wide range of non-profit research organizations which engage in public policy analysis and research, and often advocate solutions. Some are strictly nonpartisan, researching policy issues without regard to political outcomes, while others see one of their main functions as that of providing intellectual support to politicians or parties. They are as ubiquitous in the American political scene as interest groups, media consultants, "spin doctors", and the political parties themselves."
That being said, be aware that some of those listed below may be partisan and lobbying, or advocating to affect policy in some way.
Identify relevant NGOs with the following NGO directories: