A lot of American recipes use this and it is not something that is readily available in the UK. There are various different methods to make a plain flour substitute. The one I use involves cornflour and elbow grease!
Being with the amount of plain flour you need for the recipe.
For every cup full of flour you need to remove two tbsps of flour (just put back in the flour bag).
Then replace the removed amounts with 2 tbsps of cornflour.
Then comes the elbow grease- you need to sift this mix 5 times.
It sounds time consuming but it's not too bad and it does make a light airy mix that is the correct consistency of cake flour.
Then use this as you directed in the recipe.