It took me some time to realize baking powder has corn starch in it. For months, I couldn't figure out what was making me sick when I baked muffins and cookies. Then I learned about the corn starch (here's another heads up: commercial powdered sugar also has corn starch in it)! So I searched the web, Whole Foods, and my local health food stores for corn-free baking powder. Aluminum free? Yes. Corn free? Nope. So I learned how to make my own.
Homemade baking powder
1 part baking soda
2 parts cream of tartar
2 parts starch
Use whatever starch works for you. Commercial versions use corn starch- you could too, if you are not sensitive to corn, and just want to make your own baking powder for convenience, cost, or to avoid the aluminum sometimes used in commercial baking powder.
Other starches you might try, if you are avoiding corn: arrowroot, tapioca, potato, or mung bean.
Note: if you are avoiding nightshades, do not use potato starch.
This will be a single-acting baking powder, which means you have to bake your item as soon as the baking powder is mixed with liquid.