"What can I substitute for self-rising flour? I don't want to buy it because I use so little and I'm afraid it will go bad."
Self-rising flour is all-purpose flour with baking powder and salt added. To make your own, combine 1 cup of all-purpose flour with 1 1/2 teaspoons of baking powder and 1/2 teaspoon of salt.
And you are right to worry about the shelf life of self-rising flour: the baking powder will lose its potency over time, which means your baked goods won't rise as they should. Your box should be marked with a "use-by" date.