First, I would say that there is no need to be excessively concerned about
calcium. Hunter-gatherers and animals don't need to drink the milk from another
species to build strong bones, and many cultures don't use dairy, without having
a high incidence of bone fractures or osteoporosis.

The RDA for calcium seems high, but it has been calculated considering that most
of the calcium is from dairy (and only 30% of dairy calcium is absorbed).
Moreover, the average american ingests some phytates, salt, perhaps has an
inadequate intake of vitamin D and of magnesium, etc.

On the other hand, it may well be possible that wild plants are more
nutrient-dense than cultivated ones; that paleo humans got more sunshine (hence
more vitamin D), more exercise, etc. I don't know. Probably calcium supplements
are not necessary, but if you do choose to take some, then it's necessary to
balance with other minerals (like magnesium).

Now, what green veggies to eat? My philosophy is: eat whatever you like; besides
calcium, they contain many more nutrients. But to have an idea, here is the
amount of calcium (in milligrams per 100 grams) for a few foods. I didn't
include spinach (which contains a lot of oxalates). Nuts and dark green
vegetables are the richest in calcium; meats and fruits contain little calcium.

almonds           266
turnip greens     190
kelp              168
collards          145
parsley           138
kale              135
broccoli           48
lettuce (romaine)  36

Note 1: 100 g of "kelp" includes about 80 grams water.
Note 2: I forgot which foods contain oxalates (the two main ones are spinach and
rhubarb).

JL