Bicarbonate of soda mixed with cream of tartar (this mixture is essentially what baking powder is comprised of). I believe the ratio is meant to be 1:3 bicarb to cream of tartar to make baking powder. Otherwise, there are no substitutes, but using self-raising flour may help slightly (depending on the recipe).
You can add some baking powder, but it's not an ideal substitute; baking powder is a mixture of bicarbonate of soda (baking soda) and cream of tartar. This means you need to add slightly more than is baking powder than the quantity suggested for baking soda; usually around 1/4 teaspoon on top of the quantity suggested for bicarb.
You can substitute baking powder for baking soda by using 3 times the amount of baking powder as you would baking soda. Alternatively, you can use self-rising flour, which already contains baking powder. But keep in mind that the substitution may affect the taste and texture of your baked goods.
The proper substitution for baking powder is half baking soda and half cream of tartar. They both have leavening properties. If you don't have cream of tartar available, I guess I would go toward 2/3 - 3/4 baking soda and the rest cornstarch.
I'm not positive. The reverse substitution is for 1 tsp. of baking powder, you use 1/4-1/2 tsp. of baking soda and 1/2 tsp. cream of tartar. So you could try using baking powder instead of cream of tartar, but it's not guaranteed. If there is supposed to be baking soda and cream of tartar in the recipe and you substituted baking powder, that would be more likely to succeed.
Baking Powder
Baking powder in a recipe helps your item rise when baking.
baking powerder is baking powder is not the same!
baking powder
Baking soda does not rise as well as baking powder
No, baking powder does not absorb odors when used in baking.
Baking powder is an acid
baking powder is neutral