Is there a good substitute for white balsamic vinegar?

Lime juice and/or a splash or rice vinegar work great!

I make a similar salad and use apple cider vinegar. Turns out great!

It was at Perennial Pleasures Nursery in East Hardwick, Vermont...here's a link:
http://perennialpleasures.net/tea-garden-cafe

Consider trying fig vinegar as well (as opposed to fig balsamic vinegar)...had it for the first time years ago at a Phlox Festival in Vermont...our hostess served tea, fresh-baked scones, clotted cream, blackberry jam, a perfectly cooked omelette, and roughly chopped heirloom tomatoes drizzled with fig vinegar...you'll want to slurp it up like Jello...I've gone through four bottles since....

Since there are south-of -the border ingredients (corn, peppers, beans, jalapeños) you could also turn to lime juice for the acidic component; lemon juice would also work. Different from vinegars, but still worth a try.

I am in New Jersey but we are always in Lake George and that area. My wife did go to the store you mentioned. I've seen these oil / vinegar stores popping up in a lot of tourist spots around the country. I think they buy from one main distributor. They tend to be pricey but it's makes a nice gift

You could use just about anything: white wine, red wine, apple cider, champagne. I love white balsamic because it's a little sweeter. I also love it because unlike traditional balsamic, it doesn't stain everything brown, but that's purely for aesthetics. If you find the dressing to be too sharp, you could always squirt in a teensy bit of honey or sprinkle in some sugar. Hope that helps!