If you cook for your dog, there's an easy way to ensure that they are getting enough calcium

How to Get or Make Calcium Powder?

Make your own as follows: 

Egg Shell Facts

Each eggshell consists of calcium carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%), and organic matter (4%).

Each 1/2 teaspoon of ground eggshell is about 1000 mg of calcium 

Egg Shell Feeding Guide

(Please note the Feeding Guide is based on fresh food given only)

Adult Dogs:

800 to 1,000 milligrams of calcium, or 1/2 teaspoon ground eggshell, per pound of fresh food

Puppies:

1,000 to 1,200 milligrams of calcium, or 1/2 to 2/3 teaspoon ground eggshell, per pound of fresh food (450 g).

Example:

For a dog that eats about 200 grams / 0.5 lbs of fresh food per meal, each meal has to added about 1/4 teaspoon of egg shell