The best way to find a speaker or panelist on a specific subject is to find someone who has published extensively in that field. A few quick ways to find authors by subject are to:
Think tanks are another fruitful source of experts by subject. For help finding an authoritative one in the field, start with Penn Libraries' guide to think tanks. You can also Google "think tank" and "(subject of interest)" -- just be sure to vet the organization by checking their About page, evaluating their members' other affiliations, and searching for any criticism of their credibility
When looking for contact information, the most comprehensive tool is the Leadership Connect (below), which provides contact and biographical information for a wide range of people in government, business, law, media, and the nonprofit sector. It also allows for browsing by type of person (e.g. law deans), affiliation (e.g. alma mater), or industry.
In addition to Leadership Connect, consider:
When trying to find an email address for a member of government, you may be unable to locate a direct address. Members of Congress in particular hide their personal emails in order to encourage all communication to go through their websites' constituent forms. If that's the case, Leadership Library will generally have an email address for a chief of staff, main assistant, communications director, or director of scheduling, who can be contacted to forward the message to the intended recipient.
For the really difficult to find people, there are some advanced email reverse-engineering tips in this helpful LLRX article, but don't feel you need to go this far. If a person can't be located using one of the methods above, it's reasonable to pass that one on to your project supervisor for additional advice.
Finding people internationally can be a much bigger challenge than locating them within the US, since a lot of these tools will not provide information beyond US borders.
However, you can get pretty far with creative Googling. Here are a few frequently-useful strategies: