Peruse online booksellers. A number of websites (see Resources) are dedicated to the purchase and sale of rare books. These sites let you peruse listings, search based on title, price, category and age, and display detailed information about the book. In many cases, part of the reason a book is rare is due to its very history; the buyer could benefit from this information.
Take advantage of free rare book resources, both online and at libraries. Although you won't be able to obtain the book per se, you can access rare books without having to spend hundreds or thousands of dollars acquiring them. Many major metropolitan library systems or university libraries have a rare book collection. For example, the New York Public Library's Rare Book Division has over 130,000 titles. The RareBookRoom website also has titles available for free online viewing.
Attend auctions, especially estate sales or antique auctions. You may find rare books listed along with other old, rare or valuable items. Online auction sites such as eBay also represent another source.
Shop local bookstores. Some independent bookstores have a rare books section; some stores even deal exclusively in rare books. Shopping at bookstores has the additional benefit of being able to see the book up close and to ask the proprietor questions about its value and history.