If you like fizzy drinks, I’ve recently discovered a sparkling tonic called Olipop. It’s a prebiotic digestive drink. 45 calories in total, 15 grams of carbs (9 grams of fiber, 4 grams of added sugars), no fat, no protein.
Kimchi is wild-fermented with hundreds of strains of naturally occurring probiotics, and it has more flavor than sauerkraut because of the various vegetables and seasonings. (Also, most sauerkraut isn’t fermented, it’s just cabbage in vinegar.) You can try various ones to find your favorite, or just go for the cheapest option. Your gut doesn’t care what you paid. To get a sense of the variety available, you could try Wildbrine’s Korean style kimchi, Mother In Law’s kimchi (especially if it’s part of their long-fermented reserve line), and Pickled Planet’s i-boost. These all taste very different from each other.
Personally I’m a big fan of kombucha. Anything that replaces water as a healthy alternative I’m a fan of and it’s hard to find things that either aren’t loaded with sugar or artificial sweeteners. It’s expensive to buy but so easy and cheap to make. I can make 2 gallons with water, tea bags, cup of sugar to feed the SCOBY and 2-3 apples. Total time to make is about 10 days, I just keep the jars in my kitchen cabinet while the fermentation happens.