We chose to make our own balsamic syrup here. But, you can buy balsamic syrups in gourmet shops. I think they may even carry some in Costco nowadays too. Some of those, though, use a thickener instead of letting the sugars do the thickening for them. Since we want a sweetened drink, We prefer the homemade.
Keyword balsamic, black vodka, blueberry, Spritzer