Dr Christiane Northrup asks this question in her book Women's Bodies, Woman's wisdom. What a great book, so thorough and informative. I am currently reading the chapter Nourishing Ourselves with Food. Her answer to the question is dark green leafy vegetables such as kale, collard greens and broccoli. She goes on to state that most of the worlds population gets its calcium from greens. On pages 732 and 733 the DR has two pie charts. One chart shows the conventional American approach to calcium intake where 3/4 of calcium is consumed through dairy products & the other chart shows only 1/8 being consumed through dairy products. The interesting part of this second chart is the other calcium sources. A little more than half of calcium come from the dark green leafy vegetables, about 1/4 from fish, legumes and sea veggies and another 1/4 from nuts, dairy, and "other". The other is listed as mineral waters, molasses and calcium fortified orange juice and herb infusions.
This kind of information always inspires me to eat a wider variety of foods. The foods listed here also happen to be foods rich in vitamins, mineral and antioxidants. It seems like a good model to use to improve overall health. One thing I have never really gotten into is sea vegetables. I always hear such good things about the health benefits and after reading this I think it may be time to check them out. I'll keep you posted.