Baking powder is basically a blend of calcium acid phosphate, sodium aluminum sulfate or cream of tartar and sodium bicarbonate known commonly as baking soda.. By adding water to this mixture, a chemical reaction is achieved, producing carbon dioxide which is trapped in tiny air pockets in the dough or batter. Heat expands the trapped air pockets, thus expanding the overall food.
It is therefore necessary to keep the mixture dry, away from humid places.
Kitchen cupboard, larder, pantry, baking cabinet. It's what you add to flour to make sure your cakes, scones, etc, rise.
A kitchen cabinet or pantry would be my best bet.