https://www.oercommons.org/

The OER Commons and Open Education website is a hub for public domain and creative commons educational resources, like lessons plans, that are accessible to anyone, anywhere.  It is responsible for building a  community where actual teachers create the educational content, curriculum, strategies and tools.  Cutting out the middle man, publishers, makes quality education more accessible and gives more credit to the teachers. You can search the site by subject and grade, but many of the resources are adaptable across grades. After trying it I suggest having an idea of what you are looking for, since a search can produce quite a lot of results. You may have to do a decent amount of navigating around to find resources that of your desired quality or format. Its great to see education and knowledge being shared for free, which truly befits everyone out there.