To help with your final letter/paper, here are links to some resources that may be useful. In the debates over nuclear weapons, there are very few organizations out there that take as a part of their mission to actively promote nuclear weapons, thus most of the resources have somewhat of an anti-nuclear flavor, to varying degrees.
Here are several places where you can get solid information about nuclear arsenals of the world, current US nuclear policy issues, and nuclear disarmament. Consider looking into major funding sources for some of these to see what political and economic biases might be involved if that is relevant for your argumentation.
1) Federation of American Scientists This is a non-partisan research group that collects information on nuclear arsenals and publishes commentary and reviews with the Bulletin of the Atomic Scientists (see below).
2) Bulletin of Atomic Scientists This is a publication and group that tracks nuclear weapons safety issues and generally takes an anti-nuclear stance, but usually with high quality factual information.
3) Union of Concerned Scientists A lobby and advocacy organization that generally seeks disarmament and improved nuclear weapons safety.
4) Arms Control Association Another non-partisan information and nuclear disarmament advocacy organization. Good source of “fact sheets” on nuclear arsenals.
5) Nuclear Threat Initiative and related World Institute for Nuclear Security these are non-partisan independent groups that collaborate with governments and the UN to secure nuclear materials and promote nuclear safety.
6) Here are some organizations that are explicitly and wholly dedicated to eliminating nuclear weapons: ICAN (recent Nobel Peace Prize winner), Global Zero (related to a documentary film of the same name), Ploughshares Fund (which funds some of the organizations above).
7) Finally, the United Nations webpages and the IAEA webpages are excellent resources.