that calcium comes from milk and dairy products, the real source of calcium is coming from the plants the animal is consuming, the animal is just the middle man if you like.
Spinach contains relatively high amounts of calcium but also contain a substance called oxalate, which hinders absorption. It is better to get your calcium from green vegetables such as kale, broccoli and bok choy. The calcium in these is absorbed about twice as well as the calcium in cow’s milk.