Oh, it's everywhere, especially the small bags! The cheapest I've found is at Fred Meyer. Next cheapest is PCC. Haven't priced it at Whole Foods, but I'm sure it's there, too.

I buy the giant bags at PCC. It has baking soda and baking powder and salt in it all ready, so be careful it doesn't throw off your recipe.

Here's the Scone recipe on the back of the bag, to give you an idea of how they handle the additional baking powder:
2 1/4 cups Pamela's Mix
1/3 cup sugar
1/2 cup currants
1 egg, beaten
1 tsp baking powder
4 tblspn butter
2/3 cup milk

do the usual thing, bake 375 for 15 minutes, etc etc

I've never tried baking anything else with Pamela's mix, just pancakes. Though I have added it to a really bad pizza crust mix, and the Pamela's improved the pizza crust a gazillion-fold.