Cake flour

Cake flour is made from a low protein ( 6 - 8%), fine-textured soft wheat so the flour contains a high amount of starch and less gluten resulting in lighter, less dense textures or a more "tender, fine crumb" for various cake and pastry items.

Where To Buy Cake Flour

Most well-stocked grocery stores stock at least one brand of cake flour.  SoftaSilk by Pillsbury is an old well-known brand but there are many others available too.  Both King Arthur and Bob's Red Mill cake flour are recognized as reliable brands as well. 

You can find a variety of these brands online at Walmart and Amazon.com: Cake Flour

The best substitute for 1 cup cake flour is to use 7/8 cup all-purpose flour + 2 tablespoons cornstarch.
Pastry flour or substitute 7/8 cup of all-purpose flour for one cup of cake flour (product will be a bit more dense)

1 pound cake flour = 4 3/4 to 5 cups sifted