For meringue-making, I have found no acceptable substitution for cream of tartar. It is a mild acidic powder which helps to stabilize the egg whites so that they don't fall so easily. Cream of tartar is a common ingredient, even though you may not have every heard of it. It is found in the spice section of every single supermarket I have ever been to, and it is cheap as dirt, and it doesn't go bad. Buy yourself a bottle of it...it will last for years and years in your spice cabinet.
