Online platforms like Amazon or Barnes & Noble have many options. You can browse through their children's section for story collections. Another option is to check educational websites specifically designed for kids' stories.
You can try looking in your local library. They often have a wide range of story collections suitable for kids.
I think you could find such collections in bookstores. Sometimes, they organize them by age group or theme, making it easier to find what you need. Also, some authors' websites offer free or purchasable story collections for children.