Here's a recipe for making your own gluten-free baking powder from celiac.com
Baking Powder (Gluten-Free)
(Gluten & Corn Free)
1/3 cup Baking Soda
2/3 cup Cream of Tartar
2/3 cup Arrowroot (or potato starch)
Mix well. Store in an airtight container.
1-½ teaspoon of this mixture = 1-teaspoon of regular baking power