I do something similar on the sardines. I smother them in a lot of mustard and then...wait for it...I cover that in lots of currants. I know it sounds disgusting but it provides a sweet and savory taste that mostly disguises the sardines. It's the only way I can get them down.

Another option is the Season brand of Mackerel which has about 15% of the daily value of calcium. I find them much more palatable than sardines. Eating them over a large dark green salad can give you 25% of your daily value.

I bought a Krups coffee grinder that I only use for herbs and eggshells. It grinds them as fine as you would like. Bake the eggshells at a low/medium temp until they look "done" then store them in the freezer till you use. Alternatively, you can boil them. You can add them into any baked goods. I don't worry too much about the initial baking part as they are baked again into crackers/muffins. You can also sprinkle them onto foods though they are gritty. Supposedly 1/4 tsp contains about 200-250 mg of calcium so you don't need much. According to the American Egg Board, the shell of the egg is composed of calcium carbonate (about 94%) with small amounts of magnesium carbonate, calcium phosphate and other organic matter, including protein.

Another thing I put into the grinder is milk thistle seeds. They are very protective of the liver but there is research suggesting they may help strengthen bones. They are a very safe herb that can be used regularly (though check with your doc) so I bake them into my high potency crackers. They are bitter (one of the reasons they are a liver stimulant) so I add some maple syrup to balance the bitterness which also has a little calcium though "little" is the operative word.

Another great source that @mayblin suggested is Hodo Tofu. Very high in protein and calcium. Three ounces has about 260 mg.