Ground Limestone
Dolomitic Lime
Calcium Nitrate
Egg shells
Ground clam shells

But do a soil pH test, first.

Lack of even watering is #1 cause of BER

Are you growing them now or during the summer? I have a friend in the phillipines that has a difficult time growing tomatoes. Here in the US, BER is mostly cause by clay soil or inconsistent watering.... like letting it dry out then flooding the tomato.

Before adding a calcium source to your soil find out, with a good, reliable soil test, if you need that or is something else causing the problem.
Blossom End Rot's cause are not quite as simple as just adding calcium, even though the problem is lack of calcium at fruit set. The cause may be as simple as inadequate moisture at that time that keeps the available calcium from moving up the plants stem. You may have adequate levels of calcium in your soil but it is unavailable to the plants because something is interfereing with nutrient uptake, too much of something magnesium for example.

i grown them now but its warm in philippines whole year round.

how do you define inconsistent watering ? is it amount of water you refer to ? that is quite difficult to measure if you use hose to water.

Calcium sulfate ie gypsum is considered organic as long as it comes from a mined source. Recycled walboard is not considered organic.

Bone meal

Inconsistent watering is simply flooding the growing are and then letting it dry out, a fairly common practice in some areas. tomatoes need an even moisture supply because in the absence of soil moisture nutrients cannot move from the soil to the plant and if there is not quite enough moisture in the plant nutients cannot flow through the plants system. But too much soil moisture does the same because that can limit the air the roots also need to move nutrients, as well as the air soil bacteria need to function in providing the nutrients to the plants.

freshfields: gypsum for base soil, lime for acid soil, and rock phosphate for neutral soil. These are all rock powders, so should be ok organic. Don't use dolomite unless you have a magnesium deficiency. This is the most common form of calcium, but it can lead to excess magnesium -- a case where the cure is worse than the disease. Regards, Peter.

It's not uneven watering per se, it's uneven soil moisture. Heck- I watered maybe 5 times this past summer (we had good rains).

Keep the soil moist, but not flooded. Provide for good drainage (raised beds, OM) Use a good layer of mulch to keep the moisture from fluctuating.