Substitute for chicken broth in tomato soup

Just use vegetable broth. It's sold in plenty of stores, though you can certainly make your own too, lots of recipes out there. The most canonical ingredients are onion, carrot, and celery, with some assortment of herbs and seasonings, but you'll also sometimes find things like mushrooms, peppers, tomatoes (surprise!), and even root vegetables! It's hard to recommend anything in particular, though; everyone has their preferences, and stronger flavored broths may not work as well in certain recipes.

Or of course you can go find a tomato soup recipe that doesn't use chicken broth.