You can substitute all-purpose flour for cake flour if you cannot purchase cake flour or don't have any available.

One cup of cake flour is equal to one cup of all-purpose flour, minus 2 Tablespoons. Happy baking!

Furthermore, the above answer is incorrect. According to the 'Joy of Cooking,' all-purpose flour is "a blend of hard and soft wheat flours. The presence of more and tougher gluten in the hard wheat constituent results in a rather elastic and porous product... You may substitute for 1 cup of all-purpose flour 1 and 1/8 cups of cake flour."

Cake flour, however, is "made of soft wheats, and their delicate, less expansive gluten bakes to a crumblier texture. Although you will not get the same result, in emergencies you may substitute 1 cup minus 2 tablespoons sifted all-purpose flour for 1 cup cake flour."