Several places sell old physicians' drug reference books online, including Abebooks.com and online auction houses such as eBay. Antique books may also be found at antique shops, inside libraries or in the archives, in the Library of Congress or online as digital texts.
Most older books are difficult to find because first-edition books and those that are antiques can be worth a good amount of money. These may be kept by collectors, making them harder to find. Typically, the Library of Congress, which is the largest library in the world, has a copy of all texts regardless of their ages.