I'm making a stew that calls for cannellini beans but I couldn't find them at the store...?

You can use kidney beans, navy beans, "white" beans. I found that I couldn't find cannellini beans at the store since moving from CT to IA but they had "white" beans instead that were the same thing. But otherwise, beans are VERY good at being substituted for one another. Good luck with your stew; it sounds delicious!