@b_well_insomnia
Sodium bicarb with acv in soda water. I think this would only work though if you could prevent the carbonic acid from escaping, so might wanna seal it in a bottle for a bit before you drink from it, and also make sure the bottle can withstand the pressure
Nose breathing, restricted breathing in general
Bag breathing, and Haidut had a modified suggestion a while back which was sticking a tiny hole at the bottom of one of those extra large water jugs and breathing through that, remembering to keep it upright the opposite way round to allow the co2 to build up when you aren't using it. Make sure to take relaxed breaths or it can make you very light headed and lower your blood pressure considerably to dangerous levels