Milk is the best source of calcium and is also rich in phosphorus, potassium and magnesium, which will help the body absorb and use calcium more effectively. Vitamin D is also essential for the efficient utilisation of calcium.
Many other foods contain calcium, but milk and dairy products are the most concentrated, convenient sources. Its tough to get enough calcium from natural foods, if you eliminate milk from your diet. You'd have to eat 3 servings (180g) of spinach, 20 servings (1600g) of iceberg lettuce, 9 boiled eggs or 5 large servings (550g) of broccoli to get the 300mg of calcium found in one 250ml glass of semi skimmed milk!