Stinging nettles and horsetail are two plants which contain a lot of silica. Your body converts that directly into calcium. You can forage it for free or buy it in a store. Consume as tea, or in oatmeal or on salads, etc.
I wouldn't look too much for supplements, I would look more into herbs. They contain a lot and we forgot about it.
If you decide to get calcium supplements though make sure you get it in a liquid form. Tablets are often not the best thing.. some really cheap supplements actually calcify your body and you get more problems.