Concerning treaties among the U.S. and 3+ parties, The Bluebook requires researchers to cite to both a U.S.-published source AND a source published by an international organization.
Examples of treaty sources published by intergovernmental organizations are:
United Nations Treaty Series (U.N.T.S.);
Council of Europe Treaty Series (C.E.T.S., and the abbreviation for the prior European Treaty Series is: E.T.S.);
and Official Journal of the European Union (O.J.).
Regarding treaties to which the U.S. is not a party, The Bluebook requires only that users cite to one source published by an international organization. If a treaty does not appear in a source published by an international organization, researchers can cite to the treaty in the unofficial document collection: International Legal Materials (I.L.M.). Finally, if a treaty does not appear in International Legal Materials, researchers can cite to the treaty in another unofficial treaty source.
In the left frame of this web page, there are links to websites that 1) provide treaties published by international or regional bodies and 2) include sufficient information to create Bluebook-compliant cites.