What can be used as a substitute for baking soda?

The effervescence is due to the fact that carbon dioxide is being formed. Baking soda is sodium bicarbonate, and when it reacts with an acid (in this case acetic acid in vinegar) carbon dioxide is formed. This means you can use pretty much any acid you want.

In short: anything acidic, eg. lemon juice (citric acid), hydrochloric acid, sulfuric acid, etc.