It's true! I live in Krakow and have been to Carrefour and Alma, and they do not sell baking soda - only baking powder (and even that is only in really tiny little packets). Today a friend told me he saw baking soda in the cleaning section of Tesco though, so I'll try to look for it there.
Baking soda is often used for cleaning. I would be careful to review the package materials and make certain what you buy can be used in food. In the U.S we often put baking soda in the refrigerator to absorb odor. Baking soda poured down a drain followed by vinegar is a way to clean out an oil glog. The most common brand in the U.S is Arm and Hammer. My box says it can be used for baking, cleaning and deordizing. Good luck.