Start with CLIO to find books and journals in Columbia University library collections. (Use the Pegasus catalog to find books and journals in the Law Library collection.)
Then try WorldCat, the largest online catalog in the world, with over 100 million book and journal records, if you can't find what you need in CLIO. You can use Interlibrary Loan or BorrowDirect to get access to whatever you need, even if it's not available locally.
To find dissertations, start with Proquest Digital Dissertations. You can also check DART, the European e-theses portal. For Italian dissertations only, see: Tesionline.
Other essential catalogs and resources include: