Cake flour is virtually impossible to find at regular stores in Australia. It's actually a name given to a type of soft flour. Soft flours have less gluten, which usually means you'll get a finer crumbly texture; perfect for cakes and cupcakes.
By mixing plain flour with corn flour you can get a similar effect. While it's not technically the same as what you'd buy from the store, it will still help you make cakes with a better crumb.
First measure out the amount of flour your recipe calls for. Add this to one of your bowls.
For every cup of flour you have measured, remove two tablespoons of flour. You won't need to use this, so you can just add it back to your container.
Replace the tablespoons of flour with tablespoons of corn flour. Then stir to combine.
To make sure that the flour and corn flour are properly incorporated you'll need to sift the mixture at least three times.
It can be handy to make your cake flour the night before and put it in a labelled container. This way you don't have to worry about sifting and measuring the extra ingredients on the day you're baking; you can just add in your pre-measured flour mix.
Corn flour is the term used in the UK and Australia. It is not technically a flour. In the US corn flour is referred to as corn starch. Corn meal will not have the same results.