Local book stores are listed in your local yellow pages, and more distant book stores will be listed in online Yellow pages (see "Telephone Numbers"). The American Booksellers Association posts an international directory of independent book stores that lets you look up book stores location and by subject specialty.
Alternatively, there are many online book stores. Amazon.com and Barnes & Noble both have huge catalogs of print and e-books for their proprietary devices (the Kindle and Nook, respectively). The Google eBookstore sells e-books readable on most leading devices. AddALL searches many online stores at once so you can compare prices.
Legal: All law schools have bookstores, and many Barnes and Noble branches stock legal books (including Nutshells). For online buying, there's Barrister Books, the Law Book Network, the Law Book Exchange, plus the general Internet book stores.
Used Books: See the separate entry for "Used Books."