There are various guides to researching International law:
Oxford Reports on International Law
Contains coverage of International courts and tribunals, domestic courts and international ad hoc tribunals.
Max Planck Encyclopedia of Public International Law
Peer-reviewed reference work that brings together The Max Planck Encyclopedia of Public International Law and The Max Planck Encyclopedia of International Procedural Law. Includes over 1,700 articles on Public International Law and is searchable or can be browsed by subject/author
Oxford Scholarly Authorities on International Law
Cross-searchable full text of over fifty current editions of books on a wide variety of aspects of international law from Oxford University Press. Updated twice a year with additional texts added.
Westlaw International and Lexis+
Comprehensive databases for International case law and legislation. Include international judicial decisions, treaties and selected full text international journals.
HeinOnline
Provides access to the Law Journal Library (large number of international law journals in full text), Foreign and International Law Resources Database (e.g. yearbooks, digests and judicial decisions), United Nations Law Collection, and the World Treaty Library. The World Treaty Library contains United Nations Treaty Series 1946- (UNTS), League of Nations Treaty Series 1920-1944 (LNTS), and Consolidated Treaty Series 1648-1918 (CTS).
Justis (only available via IAS Library for SOAS staff & postgraduate law students)
Provides the full text of the International Law Reports from 1919 onwards.
Oxford Bibliographies - international Law Online subject bibliography which combines a research-level encyclopedia and the traditional bibliography. The bibliography includes selective lists of citations to useful print and online resources as well as narrative guides.
Index to Foreign Legal Periodicals - a multilingual index to articles and book reviews appearing in approximately 540 legal journals published worldwide.
United Nations International Law and Justice
United Nations Dag Hammarskjöld Library
Search the catalogue of the UN’s New York Library
United Nations Treaty Collection
Provides free access to bilateral and multilateral treaties, including their status and links to the full text in various languages
Council of Europe Treaty Series
Provides access to the full text Council of Europe’s treatise and explanatory reports.
International Court of Justice
Provides the text of the Reports of Judgments, Advisory Opinions and Orders and
Pleadings, Oral Arguments and Documents.