Cream of tartar is a great help in the kitchen. It increases the volume of beaten egg whites, as well as stabilizes and gives a creamy texture to desserts. (And it comes in handy for cleaning brass and copper cookware.)
I've attempted substituting other ingredients when baking Snickerdoodles, such as a combination of baking powder and baking soda, but it doesn't really do the trick. The cookies tend to flatten out and not retain their puffy, chewy texture. The only thing, it is said, that will provide the correct amount of acid when baking would be 3 times the amount of white vinegar or lemon juice, which would, then, produce other problems with your mixture.
Cream of tartar is inexpensive. It is worth spending a few dollars to enhance the variety of your spice cabinet.