Bilateral treaties to which the US is not a party are the hardest to find, and may be unavailable in English.
If possible, expand your web searches to include the name of the treaty in the language of one of the parties.
Two approaches:
1. Does one of the parties have an online treaty collection?
2. Is there a subject collection of similar treaties (e.g., bilateral investment treaties)?
Treaties Where the U.S. Is Not a Party
Bilateral Treaties
MNCAT subject heading: [country] foreign relations treaties.
The treaty series, statutory compilation, or official gazette of one of the country parties.
International Legal Materials (for major treaties).
Use World Treaty Index (P. Rohn ed. 1983) [KZ173 .R64 1983] or the United Nations Treaty Series' Index (about 10 years behind) for a citation. The full text might be available in UNTS online.
Compilations of documents from an international organization may provide information.
International yearbooks (e.g. African Yearbook of International Law) or other international law periodicals. For more yearbooks and related periodicals, use subject searches on MNCAT international law periodicals or international law [geographic area or country] periodicals.
Phone calls to interested parties (organizations), embassies, Dept. of State, Office of Treaty Affairs or the United Nations Office of Legal Affairs, Treaty Section.
Multilateral Treaties
Indexes
MNCAT subject heading: treaties indexes.
Multilateral Treaties Deposited with the Secretary General. This is a good source for citations and a list of the parties to an agreement. Limited to those treaties deposited with the UN.
World Treaty Index (P. Rohn ed. 1983) [KZ173 .R64 1983].
Christian L. Wiktor, Multilateral Treaty Calendar = Repertoire des Traites Multilateraux, 1648-1995 [Reference Office KZ 118 .M85 1998] .
Index to the United Nations Treaty Series [ KZ172 .T74]
Full-Text Sources
MNCAT subject headings: [name of individual treaty] or treaties collections.
Consolidated Treaty Series (C.T.S.) 1648-1920 [KZ120 .P35x 1969].
League of Nations Treaty Series (L.N.T.S.) 1920-1944 [KZ170.5 .T74] and online.
United Nations Treaty Series (U.N.T.S.) 1944-date [KZ172 .T74] and online.
International Legal Materials (I.L.M.) (1962-date) [Per.I577].
Periodicals or looseleaf services.
Country treaty series or official gazettes.
Subject compilations, such as Basic Documents in International Law and World Order or Basic Documents of International Economic Law, Intellectual Property Laws and Treaties, etc.
LexisAdvance, WestlawNext, and the web.