Good tools for finding used and out-of-print books include Bookfinder.com, Alibris, AbeBooks or similar databases that let you search the holdings of thousands of book stores at once.
You may also want to search some of the larger used book sellers, such as Amazon, Powell's Bookstore and/or Barnes & Noble.
Same-Day Service: The traditional way to find used books was to call or visit a local used book store, and this is still your best hope for buying a book if you need it the same day. You can find used book stores in your local Yellow Pages or in the online versions (see "Telephone Numbers"). Large used book stores in New York include The Strand (212-473-1452) and the Argosy Bookstore (212-753-4455). Both sites let you search the available stock.
Legal Materials: Used and out-of-print law books, journals and other publications are bought and sold by Law Book Exchange, Ltd. (800-422-6686), LawBooksForLess.com and, to a lesser extent, W.S. Hein & Co. (800-828-7571). Lawswap.com lets lets law students sell their casebooks, study guides and bar review materials. Meyer Boswell Books specializes in antiquarian law books (i.e., "rare and scholarly books on the law").
More used book vendors are listed in the Legal Information Buyer's Guide & Reference Manual (New England LawPress) and The Legal Researcher's Desk Reference (Infosources Publishing).
Searching for Out-of-Print Books: You can look up out-of-print books in Books Out-Of-Print, which was traditionally published as a hardcover book. Now, Books Out-of-Print is included with Books In Print on Dialog (File 470) and Lexis (BUSREF;BIP). There is also a Books In Print CD-ROM, which includes ten years of Books Out-of-Print, plus book reviews.
Alternatively, you can find bibliographic information by searching OCLC's WorldCat, and/or online library catalogs, such as the catalog of the Library of Congress or the New York Public Library.