Unfortunately, they're all terrible. The ones that aren't mostly soybean oil are loaded with corn syrup. I've looked - because I would love the convenience. But there are some that are pretty easy to make.
Blue cheese crumbles + greek yogurt + water or milk - season to taste
Olive oil + balsamic vinegar (my favorite), lemon juice, or red wine vinegar
Sour cream + salsa (I love this!)