Make your own from all-purpose flour.
Self-rising flour = all-purpose flour + 1½ tsp baking powder + ¼ tsp salt per cup. This is the standard ratio used by Southern bakers for biscuits, pancakes, and quick breads. Commercial self-rising flour uses a softer, lower-protein wheat (8-9% protein vs 10-12% for regular AP), which makes more tender biscuits. If you're making biscuits, use cake flour or a soft winter wheat flour for the closest match. Self-rising flour's baking powder loses potency over time — commercial versions include an expiration date. Homemade versions should be used within a few months. Never use self-rising flour in yeast bread recipes — the baking powder interferes with yeast fermentation and the salt level may be wrong.