Calcium supplements are available from most pet stores, often in combination with phosphorous (2:1 ration calcium to phosphorous) and vitamin d. These supplement are only necessary if you are feeding your dog a diet with very little calcium, such as a meat only diet with not bones, dairy products or vegetable matter. Most quality dog food contain more than enough calcium even for large breeds. Supplementing can actually cause problems, especially in puppies where the bones grow faster but with less density than required making them weak and fragile.