Using the United Nations Law Collection
From here, scroll down to the bottom to United Nations Treaty Series.
You can also use the “Search for a Treaty” finding aid located on the United Nations Law Collection homepage, which features several different search facets you can choose from based on the information you have.
Using the World Treaty Library
You can also find a United Nations treaty from the World Treaty Library using the UN Treaty Publications subcollection.
Or, you can use the search tool on the database’s home page. Enter your search terms, then select United Nations Treaty Series (1946-current) from the document types.