If you can't be bothered with making soup, I get a Baxters ****-a-leekie. Although it's got some rice in, it's only 5.6 grams carbs for half a tin, so no probs for a lunch or starter according to how low you're carbing. On days when I have a salad for my main meal (with meat, fish like smoked mackerel, etc) which is very low carb, I have half a can of soup for a starter.
Any "thick" soup is likely to be carby. A clear consomme might work
I don't use tinned soups any more, but I do make a "soup" from Bovril or Marmite for my lunch most days.
Hana