Tomatoes have carbs and it takes a few to make a soup with. Same with veggie soup, it's a lot of veggies in it. You could try your own and then at least control how many veggies go in it. Or which veggies, focusing on squashes, celery with some tomato in it.
Walden Farms has managed to somehow managed to make a Ketchup and Spaghetti sauce with few or no carbs. You could try using their spaghetti sauce as a base for soup? Maybe they have something else that would work even better. I'm not low carb so I just never go to their site.