There are several ways to search for ICJ opinions and supporting materials, each with a different approach:
WESTLAW has the full-text of all ICJ opinions from the beginning (1947). They are located in the library designated as INT-ICJ. Under no circumstances should you search for ICJ opinions on LEXIS. Many of the “opinions” in their database are actually press release summaries of judgments prepared by the Court!
The Court’s website: www.icj-cij.org. The court has recently revised and modernized its search engine. You can now search the full text of all its opinions. Importantly, it also has memorials and transcripts of oral arguments from the more recent cases. These can be extremely helpful, as the memorials are written by the world’s leading international lawyers. The page for each case should have a link for “written pleadings.” The easiest way to find relevant memorials is to determine which opinion of the court addresses the issue of interest and find the memorials submitted in advance of that opinion.
All the opinions of the Permanent Court of International Justice (PCIJ), the predecessor to the ICJ under the League of Nations, are available here: http://www.worldcourts.com/pcij/eng/decisions.htm. The search engine on this site is not very good. Note that the ICJ cites decisions of the PCIJ as if they were its own. So they are quite important to review.
The World Court Digest, created by the Max Planck Institute in Germany: http://www.mpil.de/ww/en/pub/research/details/publications/institute/wcd.cfm?100000000006.cfm. This is a West-like digest of ICJ jurisprudence organized according to general and specific topics in international law. It is very thorough and very helpful. NOTE, however, the Digest only goes up to 2000.
Many other international organizations have their own web sites with extensive documentation. Almost all of these are listed in the very comprehensive site of the NYU Project on International Courts and Tribunals: http://www.pict-pcti.org/links/links.html. It includes links both to international courts and their parent organizations.
Court of Justice of the European Communities: Reports of Cases Before the Court (ECR) (1954-present)
KJE 5472 .C68; Second Level; web site also available
Please see the European Union Research Guide for more information about the EU generally.