Get from this baking shop call phoon huat, refer to the url for their outlets, make sure u get baking soda and not baking powder
http://www.phoonhuat.com/sto_loc.htm
ps. i'm not related to this shop.
refer to this calculator for adding baking soda
http://home.comcast.net/~jdieck1/chem_calc3.html
the best is u add it in the water slowly and measure the kh