No cake flour at my local grocery store -- can I substitute either AP flour or whole wheat pastry flour? (They have a very random flour selection here

Four can be tricky. AP flour can vary wildly in regions of the US. Example: Southern AP flour is much lighter---Gold Medal, and White Lilly...weigh in at 130g/cup. While USDA weighs in at (memory here) at 125/g cup..and King Aurthur supposedly at 115 cup. I failed on No Kneed Bread..using Southern Flour..until I weighed it and didn't use the cup measurement.
This is way some Southern Recipes fail in the north..The AP flour used in those is either White Lilly, or Gold Medal (southern blend). Something before the internet Grandma wouldn't know about.
And just call for AP flour.

Now, a down & dirty article on types of flour would be quite appropriate it seems. Pretty please...

I don't care especially for the corn starch substitution, as it tends to result in a tighter texture than if you used straight AP flour. Go ahead and use all AP flour, but increase the butter (or other fat) by a couple ounces. Fat in any form has a tenderizing effect on protein in any form, and will compensate nicely for the additional protein you have by virtue of using AP flour.

For what purpose -- presumably making cake? You can "lighten" AP flour by with either corn starch or tapioca starch (also known as tapioca flour -- same thing) to get a better result. The standard substitution is one cup AP flour, remove two Tablespoons then add two Tablespoons corn/tapioca starch. Sift well (or mix well in a bowl with a whisk) before using. Again, assuming you want a light cake (not some other application for cake flour) your WW pastry flour will be way too heavy. And, you can always use AP flour straight across without the starch adjustment -- you will simply have a much heavier crumb.

When I run out of cake flour I do this: 1 cup AP flour -2 tbs and add 2 tbs corn starch.