You can buy roofing shakes from roofing supply merchants who carry wood roofing materials. You may, however, get a better price if you let your roofing contractor procure the shakes for you, as they can often get them at trade prices. Still, be sure to shop around to see if you can find a better deal. Just remember to check the grade of the shakes so that you don't end up with common grade when you wanted straight grain.