Calcium carbonate works as well as baking soda. If both GH and KH are low I'd add CaCO3 to get both up. CaCO3 will raise both an approximately equal amount. Once the GH is where you want (target 4 or so) then add baking soda if the KH is still a bit low (also target 4 or so).
CaCO3 is available in tablet form in drug or nutrition stores as a dietary supplement.