There's quite a few places where you can get the source material!

This includes the national archives and the British Library. Both of these are quite UK based, but other countries will have similar archives. For Germany I'd recommend checking out their archives. There's also the commonwealth war graves but this is more directly linked to locating gravestones of family members.

Specifically on video material there is Imperial War Museum's archive and the national archives, there is also the European film gateway.

When watching these films remember there would be reasons why they were filmed, video cameras weren't widely available so most of the recorded footage is propaganda. Such as the famous nazi propaganda movie Triumph of the Will . So keep this in mind when viewing the material.

I hope this helped. 😊