At the beginning of the public international law collection, & along the northern flank of the Wing of the Upper/Gallery Floor of the LawBod, with shelf marks Internat 5 to Internat 90 are various collections of treaties, and some finding aids.
You are most likely to need one of the following:
Consolidated Treaty Series (CTS) 1648-1918 is available on open shelft Internat 5
League of Nations Treaty Series (LNTS) 1920-1946 is available to order to the Bodleian Law Library. The General Index is available on open shelf at Internat 010 LN
United Nations Treaty Series (UNTS) available to order to the Bodleian Law Library
For the texts of recent treaties not yet published in UNTS you will find International Legal Materials (ILM) at Internat 300 I240
Members of OU can access the text of treaties (LNTS, UNTS and selections from ILM) electronically via the links below.
The ETS: European Treaty Series is shelved at Internat 25, but is also freely available online for the Council of Europe website.
The EISEL : ASIL Resarch Tool (the last link below) is a safe way to find free, online sources for the text of treaties.
If you need to cite to the text of a treaty, OSCOLA makes the following distinctions
Sources of the first rank
For post-1960 treaties not yet in one of above cite to International Legal Materials (ILM)
Article 38(1) of the Statute of the International Court of Justice tells the court to decide cases before it by applying
"(a) international conventions, whether general or particular, establishing rules expressly recognised by the contesting States;
(b) international custom ....
(c) the general principles of law recognised by civilised nations
(d).... judicial decisions and the teachings of the most highly qualified ... as subsidiary means for the determination of the rules of law."