Various research guides, books and articles provide lists of the types of state actions and documents that might be evidence of "state practice" and/or opinio juris. See Research strategies & selected books, e.g., Kuehl & O'Brien, International Legal Research in a Global Community, at 76.
Use the other tabs of this research guide to locate any specific sources that you need, e.g., U.S. practice of international law, treaties, case law/jurisprudence, international organizations, etc.