what can i use instead of baking soda

Well you are best off using both when they are called for. However try increasing the amount of baking powder at the rate of 1/2 tsp. for 1/2 tsp. Sometimes you can substitute some of the liquid with soda water, but I think the baking powder is a better idea.
What kind of a recipe is it?
Hugs Squirrelly Cakes

Well, that is one thing I wouldn't substitute the baking powder for, they will rise differently and be more cakey than cookiey, how is that for technical. Not a good thing to substitute with, unfortunately.
Here is my absolute favourite oatmeal cookie recipe. It too uses baking soda, unfortunately.
Oatmeal Raisin Cookies
1 1/2 cups butter, softened
1 1/2 cups packed brown sugar
1 cup white granulated sugar
2 eggs
4 tbsp. water
4 tsp. vanilla
1 1/2 cups all-purpose flour
1 1/2 tsp. baking soda
2 tsp. cinnamon
6 cups Robin Hood Old Mill 100% Naturally Nutritious Large Flake Oats
*You must use these oats to get the best cookies
3 cups seedless raisins, I use Sultana
Cream butter, add sugars and continue creaming on low speed. Add eggs, water and vanilla creaming until light and fluffy. Combine flour, soda and cinnamon and add to creamed mixture. Mix on low. Add oats and raisins, mixing until well incorporated on low speed. Drop by heaping teaspoonfuls onto a greased baking sheet. Bake about 10-15 minutes at 350F. Leave rounded for chewy, flatten for crispy cookies. This is a recipe that you let sit for a couple of minutes on the cookie sheet before removing to cooling rack. Recipe can be halved.
Hugs Squirrelly Cakes

ARM & HAMMER® Baking Soda is 100% pure sodium bicarbonate. Baking powder is a mixture of baking soda and various acidic ingredients. Both baking soda and baking powder are used in baking to help dough rise, (for leavening). ARM & HAMMER® Baking Soda will react in combination with acidic ingredients in a recipe (like cream of tartar or lemon juice) to release the carbon dioxide that causes dough to rise. Baking powder, which already contains the acidic ingredients to make the carbon dioxide producing reaction, is used in recipes without other acidic ingredients.

As they are fundamentally different products, you may not substitute ARM & HAMMER® Baking Soda for baking powder in a recipe. You can, however, make a baking powder substitute using ARM & HAMMER® Baking Soda. To make the equivalent of 1 teaspoon baking powder, mix 5/8 teaspoons of cream of tartar with 1/4 teaspoon ARM & HAMMER® Baking Soda.

Hi again,
If the recipe called for 1 tsp. of baking powder and 1/2 tsp. baking soda, I would have just doubled up on the baking powder or added 1/2 tsp more, if it was a cake and I was really desperate, haha! But for cookies, I wouldn't even try.
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How much baking powder do I have to substitute for baking soda?
Baking soda is four times as strong as baking powder so if your recipe calls for 1 teaspoon of baking soda, you would need four teaspoons of baking powder to produce the same amount of lift. Unfortunately, though, its not that simple.

Baking powder is made of baking soda and exactly the right amount of acid to react with the soda (it also includes corn starch to keep the ingredients from prematurely reacting in the privacy of their container). So if your recipe already has acidic ingredients that were going to neutralize the baking soda called for, you are adding other ingredients that may not sit well with them.

Substituting for a lack of baking powder is very easy: 1/4 teaspoon of baking soda plus 1/2 teaspoon cream of tartar (ignoring the cornstarch) for each teaspoon of baking powder required.

Weve looked in dozens of books to be sure, but no one provides information for the reverse procedure substituting baking powder when you dont have soda on hand. To do so, you would have to consider the acidic ingredients in the recipe, and perhaps reengineer the recipe to replace them with more neutral ingredients (using whole milk instead of buttermilk, perhaps). But at that point, you would see, it would be easier and probably a lot more successful to pick up a box of baking soda.