There are other sources of calcium, but milk and dairy products, including lactose-free dairy, offer the best payload, followed by various calcium-fortified products like orange juice and some cereals. Without them, it can be hard to get enough calcium.
A cup of rhubarb has 348 milligrams of calcium, compared to 302 milligrams in a glass of skim milk. A cup of frozen turnip greens has about 249 milligrams, and a cup of frozen kale has 179 milligrams, while 3 ounces of canned salmon with bones has about 181 milligrams. If you don't eat dairy, ask your doctor about getting more calcium in your diet and whether you should take a calcium supplement.