You can use regular grocery store balsamic vinegar, but I would highly recommend reducing regular balsamic so it becomes thicker and more syrupy, similar to an aged balsamic. This will help with the acidity, too. Take 1/2 cup and bring to a boil, then reduce to a simmer for 10-15 minutes until reduced to half. Allow to cool (it will thicken more as it cools, too), then store in an airtight container.