These places offer collections of various media and allow you to use it for educaitonal purposes, with attribution.
Remember, you can create props and photograph images yourself or you may stage short scenes (excerpts) from plays and video them. Also, if you come accross an image from an institution (a museum, university, etc...) or a place with contact information, you can ask them for permission to use images and content. Be sure to keep evidence of permission, if it's granted.