Your local library might also be a good place. Some libraries offer printable materials, including kids' stories. You can ask the librarian if they have any such resources available. Additionally, some children's book publishers' websites may have printable stories for promotional purposes.
There are also specialized kids' learning apps that may have printable kids stories as part of their offerings. For example, ABCmouse might have printable content. You can check in the app's resource section to see if there are any printable stories that you can use for your kids.