Balsamic Vinegar

I just came across a couple of recipes for fresh string beans that said to use 1/4 cup balsamic vinegar (and 1/2 cup dry sherry) in the cooking process. Now, we have some balsamic, but it is in small bottles, and is very expensive. We only use a teaspoon at most to any dish. Is there a cheaper grade, along the lines of olive oil, that I don't know about? If so, is it in the regular grocery stores? Any brand better than the next?