My recipe calls for baking ammonia but I can't find it at the supermarket.  Where can I find it?

Baking ammonia, also called "Hartshorn" or "Hart's Horn" is ammonium bicarbonate, a leavening agent. It can often be purchased in pharmacies. It is often the leavening called for in German Springerle cookie recipes. If you cannot find baking ammonia, The Joy of Cooking recipe book has a Springerle recipe that uses ordinary baking powder.

Barbara Willenberg, Nutritional Sciences, University of Missouri-Columbia.