Cake flour it's a mixture of AP flour and cornstarch to minimize the glutain formation, it gives the cakes a more crumbly texture and it's less likely to toughen the batter ( You can do the same with AP Flour most people use this, just need to beat less) I personally use cake flour and to me it's very cost effective, if you buy it in bulk. I paid $12.00 for a 50 Lb bag at a local supplier. Swanson has a box of 2 lbs for about $2.50 ... I guess you can google local cake flour wholesalers and call them and see if you can buy from them.. God luck.. HTH
Anna
Here is a comparisson:
If a recipe asks for cake flour specifically, it is better not to substitute with any other flour but general substitution formula for 1 cup of cake flour is equivalent to 1 cup of all purpose flour minus two tablespoons. Some also say you can add about 2 tablespoons of cornstarch to the all purpose flour.
Summary:
1.The major difference between the both flours is the amount of protein present in them that influences the gluten or elastic quality of the flour. Cake flour has low protein content, around 7-8% while all purpose flour has around 11-12%.
2.Cake flour is made from soft wheat whereas all purpose flour is a combination of both hard and soft wheat.
3.Cake flour is finely ground and chlorinated sometimes as against all purpose flour
Substitute:
Cake flour has less gluten than other flours, such as all-purpose, and because of this items baked with it tend to be lighter and more tender. If you ever have a flour recipe that calls for cake flour, but all you have is the all-purpose stuff, you can make a simple substitution.
For every cup of cake baking flour called for, put two tablespoons of corn starch in the bottom of your cup measurer, and then fill it up with all-purpose flour the rest of the way. The corn starch, which has essentially no gluten, will cut the gluten content of the all-purpose flour and make it more like cake flour. This substitute works better for denser cakes and breads that call for cake flour than for other items such as angel food cake, which really requires cake flour in order to be light.
HTH
Resources:
Read more: Difference Between All Purpose Flour and Cake Flour | Difference Between http://www.differencebetween.net/object/difference-between-all-purpose-flour-and-cake-flour/#ixzz0mluSdnnS
http://www.life123.com/food/baking/baking-tips/cake-flour.shtml
MammaG where are you - here in he uk we don't have cake flour and our flour is not chlorinated but there is a way to make it and it works a treat
Cake flour it's a mixture of AP flour and cornstarch to minimize the glutain formation
Actually cake flour contains less of the proteins glutenin and gliaden, which form gluten. The misunderstanding likely comes from the fact that a common substitute for cake flour is replacing a portion of AP flour with cornstarch as mention above, but this is not what commercially available cake flour is.
I find my boxed cake flour in the baking aisle of my local grocery stores and in my walmart supercenter grocery section. It's usually on the top shelf surrounded by other types of flours -not hard to find at all 
Cake flour it's a mixture of AP flour and cornstarch to minimize the glutain formation
Actually cake flour contains less of the proteins glutenin and gliaden, which form gluten. The misunderstanding likely comes from the fact that a common substitute for cake flour is replacing a portion of AP flour with cornstarch as mention above, but this is not what commercially available cake flour is.
I agree with you, At the moment I posted this I had in my mind the recipe for the substituting formula.... I stand corrected 
so using cake flour gives cakes a more tender texture? probably not good for stacking and carving?
would using regular AP make a sturdier cake?
Almost all of my cake recipes use cake flour, and it's never affected their ability to be stacked or carved. You should use whatever your recipes call for, as recipes are usually formulated based on a certain type of flour.
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