Sodium Carbonate is sold as washing soda at the grocery store. Sodium Bicarbonate is the same thing as baking soda - it will drop your PH a bit (for like a few hours) as it releases some CO2, but you can bake it at 450 for a few hours and drive off all the CO2 and then it becomes Sodium Carbonate. Both washing soda and baking soda are food grade. There is no need, ever, to buy something made especially for the reef, IMO.
I just use baking soda if I need anything. Cheap, easy and effective.