A useful print resource to use to find state practice is:
For assistance in researching the law of particular jurisdictions, see the Library of Congress Guide to Law Online - see Nations of the World
As the Restatement says: "Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation."
Customary international law may be found articulated in multilateral and bilateral treaties; actions taken by States through international organizations (e.g. resolutions of the U.N. General Assembly); decisions of international tribunals; domestic court decisions and national domestic legislation; diplomatic correspondence and administrative decisions. The age of the material you are looking for will, in many cases, determine whether or not you can find it online.
Look at the resources the UCONN Law Library has listed below to find these sources of Customary International Law.
For information on the the state practice of international law, look for country specific digests and international law year books (sometimes spelled yearbook)
For instance, the UCONN Law Library has the following (selected):
Law Library does not have complete holdings of many of these titles in print. Most of the electronic editions are on HeinOnline (Net Id may be required).
Find international actions taken through organizations at those organizations' websites.
Also look at NGO and international organization sites for databases of laws and conventions regarding particular topics.
To find books on Customary International Law in library catalogs, including UCONN Law Library's online catalog, use the LC Subject Heading: Customary Law, International
Here are selected recent books - dealing more with the theory rather than the rules of Customary International law - in the UCONN Law Library:
To find books on the topic that the UCONN Law Library does not hold, use WorldCat (NetID required)
Use the LC Subject Heading: Customary Law, International to find relevant articles in periodical indexes and UCONN Law Library's research databases [NetID may be required] such as, Hein Online and JSTOR.