You can drop upwards of $40 on a bottle of balsamic reduction, or, buy a cheap bottle of balsamic and make your own, with better flavor.
Trust me, make your own.
Put your frying pan on a burner turned to medium low heat, letting the pan warm.
Dump the entire bottle of balsamic into the pan.
Cook low and slow until the balsamic has reduced down to AT LEAST half of it's original volume, and coats the back of a spoon.
Let it cool completely, transfer to whatever bottle you're using for storage, and put in the fridge until you're ready to use it.