Ascorbic acid will work, but is going to be a lot more expensive and is very instable. I would go with citric or malic acids, which are only about $4 a pound. And a pound will go a long, long way. If there is a wine or beer making supply close by you can get these there. Citric acid is also available in Indian markets, and cooking supply or possibly craft stores in the candy making supply section.
I have always heard that baking soda is too abrasive and can damage the enamel. I guess using a soft bristle brush will help if using abrasive tooth cleaners.