If you do decide you need calcium supplementation, the cheapest (and possibly best) way is via powdered eggshell. The eggshells should be boiled (to remove pathogens on the outside - clean, boiled-egg shells are ideal), dried, broken up, and can be powdered in the coffee-grinder attachment of a blender.
This gives you powdered calcium carbonate.
I used to dissolve it in lemon juice to make the citrate, but you can just drink it down in warm water as it is (you shouldn't need much, e.g. less than half a teaspoon per day). Might need a few additional swigs of water to get it all down.
People say that calcium carbonate is less bioavailable than other calcium salts, but not everyone egrees. The following is a sales plug, but I've seen non-commercial sources saying similar things:
And (if you eat eggs anyway and have a grinder), it doesn't cost you anything, and you could "afford" to take more of this (if necessary), than you would of an expensive commercial supplement.
I agree with the vitamin & mineral reccomendations (from food sources where possible). Also have it with a nice fat-rich meal.
This is one non-commercial source talking about calcium supplementation, and doesn't seem to have a problem with the carbonate form, although it says citrate is better for certain individuals.
This says much the same:
(For anyone using milk as part of their calcium intake, then according to Barry Groves, it should be full-cream milk (whole milk), in order to help absorb the calcium. (And I would say it should be unhomogenised if you can get it; even better if it's raw, if you can get it).