If you don't have any cream of tartar in your pantry, you can use one of these substitutes in its place:
For beating egg whites - use an equal amount of white vinegar or lemon juice, or omit the cream of tartar entirely
As a leavening agent - replace the baking soda and the cream of tartar in the recipe with baking powder. One teaspoon of baking powder replaces 1/3 tsp of baking soda and 2/3 tsp cream of tartar
For frosting - leave the cream of tartar out. No substitute is necessary.