I would check out the Digital Public Library of America: https://dp.la/

Although, I would assume their licensing agreement says you can use it if you leave the water marks on.

The other thing I would check would be various state's historical societies. Mine has a big digital collection. But, once again, I'm pretty sure they allow use of it for certain things but you have to keep the water mark and credit them.

I'm not sure how much stuff they have online, but there's also the National Television and Radio Museum. https://ncrtv.org/