Thanks guys - I always wanted to have this discussion. Honestly, my best source for old footage is YouTube. The Sneaker game highlights have been up there for years and I have a few other favorites.
I've also made some (granted not persistent) attempts to track down old footage myself. I called the HOF and they told me they have about 30 minutes of film from the 30s available and that if I made an appointment I should be able to watch it. Someday, I would like to do that. But, when I questioned a little further, the gentleman told me that the HOF really isn't in the business of selling footage to "fans" and that I should call NFL films.
When he mentioned NFL films, I thought of Chris Willis's bio of Dutch Clark. It mentions four pieces of footage on Clark. I went to the NFL films website and found this:
http://www.nflfilms.com/specialorders/. It didn't sound that encouraging, nor list anything from the era I was interested in, so I stopped there for now. I'm wondering if it's possible NFL films considers some of this footage to fragile to transfer to DVD - I don't know the technicalities of it.
So hypothetically, say someone was selling Dutch Clark footage or whatever and the NFL or NFL films had the rights - it would be illegal for me to buy it, but there may not be a way for me to buy it from the owner of the copyright - that feels kind of.... frustrating.