They don't really tend to sell cake flour here in Britain, but it is quite easy to make your own cake flour at home. You need all-purpose flour and cornflour aka cornstarch. Take your one cup sized measuring cup and add 2 tablespoons of cornflour then fill up the rest of the cup with all-purpose flour and level off. Then dump the cup of flour into a sieve and sift. Obviously if you need more than 1 cup of cake flour for your recipe just repeat the procedure.