The most commonly used base is potassium hydroxide.

Acids:
Nitric Acid
Phosphorus Acid
Citric Acid

You can use potassium hydroxide, sodium hydroxide or baking soda to raise pH level while phosphoric acid or sulfuric acid can be used to lower pH. Just make sure that you add acid to water, never the opposite. You can check with your local hydroponics store if they are selling these substances in bulk and cheaper amounts.

How advisable is baking soda. Is it as good for the plants as potassium hydroxide. I guess I use it for occasional heartburn so it shouldn't be too bad. Baking soda is cheap (like me).

Mark