Measure the size of the book including height, width and thickness.
Look for reproduction dust jackets that are designed to fit your size of book. There are many reproduction dust jackets available. Often interior decorators will add a color coordinated dust jacket cover to books in rooms where the display of books can look cluttered. Dust jackets are sold by the range of sizes the cover will fit.
Purchase original dust jackets by visiting a local used book store and placing a request for the specific book cover that you are missing. There is a used market for original book covers. Prices for covers vary depending on the rarity of the book and the condition of the cover. It is a good idea to bring the book with you to the store so that the clerk can identify the exact publishing information for your edition.