I buy it online when I'm buying other King Arthur products, but I tend to use it all before it's time to place another order. Oh well ...
The good news is that self-rising flour isn't a magical or complicated product. It's easy to mix it up yourself. Sure, you won't get the benefit of the softer wheat flour, but that's not going to kill a recipe. If I'm not planning on making multiple recipes that require self-rising flour, I mix just enough for a recipe or two.
When I know I'm going to be making a lot of biscuits and similar goods over the next week or so, I mix up larger batches. This stuff doesn't go bad quickly, so you don't need to fret about leftovers that hang around for a few weeks or even a few months.
Want to make some? Here's how:
For a small batch of self-rising flour:
2 cups (9 ounces) all purpose flour
1 tablespoon baking powder
3/4 teaspoon salt
Combine all ingredients and whisk well to distribute the baking powder through the flour. Or, if you prefer, you can sift it together. Store the same way you store your flour.
For a large batch of self rising flour:
1 5-pound bag all purpose flour
1/2 cup baking powder
2 tablespoons salt
Stir or sift the ingredients together well so that the baking powder is evenly distributed in the flour. Store the same way you store your flour.