The Institute of Advanced Legal Studies (IALS) in London serves all universities in its role a national legal research library.
A wide range of freely available online research and training guides are available.
Recommended resources include:
#LawPORT A collection of free-to-use training resources covering:
Customary international law
Treaties & international conventions
Judicial decisions in international law
IALS Subject Guides
IALS Jurisdiction Guides
IALS International Law Guides