You can combine all-purpose flour and cornstarch as a substitute. Here's how to do it: take one level cup of all-purpose flour, remove two tablespoons, and then add two tablespoons of cornstarch back in.
Be sure to sift the flour to distribute the cornstarch well before using it in your cake batter. When added to all-purpose flour, cornstarch will inhibit the formation of gluten while also giving structure and 'sponginess' to your cake.
The primary difference between cake flour and all-purpose (AP) flour is the protein content (which becomes gluten). The protein content of cake flour is about 8%, while the protein content of AP flour is about 10-11%. When you're making cakes, you want to use a flour that's lower in protein. It should have a little gluten for structure but not so much that it gets tough and chewy.
You can also check these baking hacks from Cosmopolitan.com.