Is there a substitute for white balsamic vinegar? I've never seen this product.

Sure thing! You could use cider vinegar, rice wine vinegar, white wine vinegar, champagne vinegar. I love white balsamic because it's a little sweeter than others, so you may have to adjust the sugar to taste. Also, ask your grocery if they carry white balsamic. I usually see two brands: Colavita, which I love, and Alessi, which is also very good.