I’m planning on baking lemon bread, but I’m out of baking soda AND baking powder. I can’t seem to find a good substitution for baking soda that doesn’t involve baking powder. Does one exist?
Short answer: no. Plus there's not enough info on the kind of bread you're making (Sweet? Savory? Does it have eggs?) to really give an answer. If it were me, I'd run out and get some baking powder and soda without hesitation. I would not bother with work arounds for such an important ingredient.
I think there are things they use in commercial baking, but nothing you're likely to have around the kitchen.