Here is another alternative baking powder I found with slightly different measurements. Already made baking powder, www.hainpurefoods.com has baking powder called Featherweight Baking Powder and it is made with potato starch instead of arrowroot, and is gluten free. I have used it but found that I can make my own cheaper and keep it fresher.
1/3 cup baking soda,
2/3 cup cream of tartar
2/3 cup arrowroot starch.
Blend together baking soda, cream of tarter and arrowroot with a wire whip in a 2 cup measuring cup. Store ingredients and label expiration date on a clean glass jar. Store in dark dry area for a month. Try starting out with equal 1 to 1 ratio for exchange and adjust from there.
Source: Living Without February/March 2011 page 68