Know exactly what you're looking for before you begin your search. Having the complete title, author's name, edition number and publication year will help you locate the book.
Look in Books in Print to see if the book is still being published.
Lurk at garage sales, antique stores, estate sales and bookstore sales, and scan book dealer catalogs.
Check out eBay's and Amazon's auctions as well as some of the larger used book sites. Some Web sites have a search option that checks many small stores across the world.
Decide what condition is acceptable for your collection. If you're a collector, you'll generally want first editions and well-cared-for volumes.
Look for books printed on early presses or with wooden type. Woodblock pictures indicate a good investment, as do hand-colored illustrations and first editions that have been signed by the author.
Focus on books concerning art, diaries and atlases if you're looking for items that will appreciate in value. Bibles and other religious books generally do not.