Free Royalty Free Stock Footage
There are several websites that do that. For example, this one accumulates free stock photos and there's also a video section. However, your best bet is to use a designated search engine like the creative commons search site, which will allow you to find material distributed under creative commons licenses. Before you use any of the found material, make sure to check which license it's using - some of them only allow non-commercial usage or require you to share any derivatives under the same license.