The following three (although the first two are no longer being updated) Guides to the basics of International Environmental Law research are recommended as starting points:
American Society of International Law Research Guide to International Environmental Law, although not updated since April, 2015, uses a well-organized narrative format and includes an introduction/overview of international environmental law research, search strategies, and long, descriptive lists of primary and secondary sources.
The GlobaLex Basic Guide to International Environmental Legal Research also uses a clear, well-organized narrative format and includes an introduction, starting points, and key abbreviations. Treaties are organized by topic, with links to the web site for each treaty. It includes links and descriptions of indexes and finding aids for treaty research as well as to the resources for national laws relating to environmental law, suggestions for searching library catalogs, and a great deal more.
International Environmental Law Research Guide by UC Hastings Law Library, also well organized and straight-forward, provides a brief introduction; current awareness resources; a list of relevant subject headings; online databases, many of which are available here at VLS; search tips specific to international environmental law research on Westlaw, Lexis Advance and Bloomberg; free internet resources, many of which are listed and described in ELRS..
Other sections of this Environmental Law LibGuide will also be helpful when searching for international environmental law resources, since resources with an international environmental focus will, of course, be found in general environmental law sources. The resources listed below will be especially helpful. And don't forget the extensive collection of international environmental books that you can locate through a JULIEN Catalog Search.