I was recently investigating how to make my own baking powder. I wanted a plain baking powder, without all the additives - one that was Gluten Free, Aluminium Free & Preservative Free... It turns out it's quite simple to make your own.
I've put together a summary of basic information on Baking Soda, Baking Powder and SR flour as well as some handy reference guides to keep in the kitchen. They can be found here on the craftnhome website.
Given that Baking Powder is easy to make, and expires anywhere from within 3 months to a year - it makes sense to make your own when you need it.