Assuming you don't cheap out and buy it at your local supermarket, assuming you can do that, here is a recipe to make it yourself:

• 1 garlic clove, unpeeled
• 1 tablespoon prepared Dijon-style mustard
• 3 tablespoons balsamic vinegar (Available at specialty food shops)
• salt and freshly ground black pepper, to taste
• 1 cup best-quality olive oil

1. Cut garlic clove into halves and rub the cut sides over the inner surface of a small bowl. Reserve the garlic.

2. Whisk mustard and vinegar together in the bowl. Season with salt and pepper to taste.

3. Dribble oil into the bowl in a slow steady stream, whisking constantly, until dressing is creamy and thickened and all the oil has been incorporated.

4. Taste and correct seasoning. Add reserved pieces of garlic; cover the bowl and let the dressing stand at room temperature until you need it. Remove garlic and rewhisk the dressing if necessary before using.

Makes 1 1/4 cups.

Despite not being an accomplished cook, may I suggest that you add two small items to an otherwise superb recipe? Firstly, a little lemon juice (juice of quarter of a lemon). Secondly, specifying salt as sea salt (not table salt).
I find that the lemon tends to pick the vinaigrette up a little and balances some of the mustard. I suppose it's a matter of taste.