If you're like me, you don't have very much pantry space. If you're like me, you also loathe buying gimmicky specialty ingredients. So, when a recipe calls for cake flour, I stick with my plain all-purpose and some good old-fashioned cornstarch. Very easy, economical, and it doesn't take up any extra cupboard space!
There are lots of tips out there for how to sub all-purpose flour for cake flour, but here's what I do: Put 2 T cornstarch in the bottom of a 1 cup measuring cup, then fill to the top with flour as normal and level with the back of a butter knife.
If you need 1/2 a cup, put 1 T cornstarch in the bottom of a 1/2 cup measure and proceed as above. You're all smart; you get the idea! You'll have to use a little more math for more difficult measurements, but as I said, you're all smart. (If you have homeschoolers, let them help you figure out the ratios!)