Substitute for baking soda low low sodium for baking
Before modern baking soda, Ammonium carbonate (baker's ammonia) was used as a leavening agent. 
You should use it with caution though, it is not only toxic but also might impart an unpleasant smell if what you’re baking is moist or thick. It’s better to use for thin dry baked goods like crackers and cookies. So that the gasses can completely escape.

Another alternative is Potassium Bicarbonate, most recipes call for 1:1 substitution. You can expect similar results as for leavening, it has a bitter aftertaste though.