Each country party to a bilateral (or multilateral) treaty or agreement keeps an official copy of the treaty in their official language(s) as part of their official records. While some countries have published print series of their treaties, many do/did not and so texts are/were difficult to find.
Use the indexes and guides at the Home tab to help you find these series. WorldCat can also be used to locate a print treaty series for a particular country or region.
Increasingly, however, governments (as well as regional organizatons--see mulitnational treaties & agreements box) are putting their treaty texts online. You can use the International Treaties Collection on Worldlii to find official electronic versions of national treaties. Be aware, however, that what is online may only be selected texts or only the texts since a certain date.
See, for instance:
Treaty documents 1965-present (U.S. Senate - Congress.gov official website) Treaties submitted to Senate for ratification. Once you identify the text you want, you use link at bottom of page to go to GPO site where full text is (1975-current; limited text and information before 94th Congress). Note the search includes treaties not yet ratified by Senate.
Treaties Pending in Senate updated to 2015 (U.S. State Department official website) Links to official texts of treaties submitted for ratification but not yet ratified by Senate.
International Agreements database 1982-current (U.S. State Department official website) Note that you are provided link to FOIA site where texts from 1982-2005 are to be found.
Treaties and Other International Acts Series (TIAS) 1996-current (U.S. State Department official website)
See other information about finding treaty texts at the U.S. Department of State, Office of the Legal Advisor
HeinOnline Treaties & Agreements Library - Includes texts of treaties, treatises about treaties and other helpful material for doing research on U.S. treaties.
Westlaw U.S. Treaties and other International Agreements - texts of treaties 1798-to present - A good wat to access from home page is by clicking Administrative Decisions and Guidance, then click on All Federal Administrative Decisions and Guidance, the click on Department of State and click on United States Treaties and Other International Agreements. If you are going to use the treaties database frequently, it is a good idea to save it as one of your Favorites, so you do not have to access it this way again. Note that you can also select Native American Law Treaties from this screen, NAFTA and US Treaties in Force.
Official texts of multilateral international treaties are each kept by the body which serves as the secretariat for that treaty. Use the various treaty indexes in Indexes and Guides at the Home tab to help you locate treaty texts.
You can also find texts of treaties and agreements on websites of NGOs and International Organizations which have an interest in a particular subject matter.