See Bluebook Rules 21.9(c) and 21.4.5(b).
Some of the following citations also include U.N.T.S., I.L.M. or other sources not required by the Bluebook but widely available online or in libraries.
Accession treaties and external treaties can be searched using the Treaties search engine. Accession treaties are listed in the Treaty dropdown menu. For external treaties, select the Collection "International agreements" and use the other country's name in your search.