I believe some have bacteria that are beneficial in other areas of the body (kinda like acidopholous helps with yeast infections... actually, there are some marketed for urinary health). I've gotten some online, but the latest batch was from petCo (convenience in the moment). Chewy.com has several to choose from. I've used Ark Naturals, but have since looked into something I can buy in larger quantities because I have 5 animals to give it to. The vet said most probiotics available are beneficial, and that he hasn't seen much difference in the ones you can get online vs the ones he sells. Generally, the higher the active bacteria specimen count in the probiotics, the more expensive it is. I've not found any additional benefit to the $80 ones over the $7 though. It might just be benefits I can't really see.

They also sell urinary tract health supplements. I have not tried those for my old cat yet because he's super picky about what he eats and they only come in treats.