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.
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