Just mix together 1 tsp of baking soda, 2 tsp of cream of tarter and 1 tsp of tapioca starch (you could probably use arrowroot also).
I tested this out over the weekend and it worked wonderfully! You could leave out the tapioca starch if you need to but be sure to use it immediately. For longer storage, you need the tapioca starch to absorb any moisture.
This is such a great option for those of us that are corn free but also is a great money saver! Most commercially available baking powders have aluminum in them so this is a great aluminum free option.