The LawBod's collection of public international law journals is at Internat 300.
The arrangement is then alphabetical, by the first word of the journal's title.
Yearbooks can be an exception - eg. yearbooks of the UN & its organs may well be in Official Papers on Floor 0
If you cannot find a journal browsing the shelves, do a full title (of the journal, not that of a particular article you wish to read in it) search in SOLO
Unfortunately, there is not one tool to answer the question quickly.
Final recourse would be to check the contents of the major databases listed below the link for Cardiff Index, in case they have recently added journal titles to their holdings.
Each year the Registry of the ICJ issues a Bibliography listing such works and documents relating to the Court as have come to its attention during the previous year. This includes articles in journals & chapters within books.
There are special tools - make use of them!