When searching in Google or another search engine, using a country's specific government domain extension can help you restrict your results to sources from a particular country's government websites.
For example: "climate change" AND site:.gov would show you results from U.S. government websites that mentioned "climate change".
The UN Official Document System (ODS) provides access to the full-text of UN documents including those created by the "Security Council, the General Assembly, the Economic and Social Council and their subsidiaries."
The United Nations Digital Library is a catalog of UN documents and publications, with and without the full-text available.
Visit the UN Documentation research guide to learn more about UN document symbols as well as the types of UN documents that are available online.