When it comes to extant tribes, many of them have web pages with info about them. The depth of information varies from tribe to tribe, I think typically encapsulating whatever the tribe feels comfortable sharing publicly. However when it comes to extinct tribes, much of what you'll find will probably be spotty and questionable, as what is known is likely the result of archeology and accounts from nearby tribes.

It's really frustrating how difficult it is to learn about the native cultures as someone on the outside. It gets glossed over in school and what you hear in pop culture is often heavily skewed or butchered to put on a good show for the audience. Then, because of how much of it gets butchered, chopped and screwed, the people who actually know the real stories become understandably protective and reluctant to share them. It'd be nice if there was a central, wikipedia-like site run by the tribes where you could learn about their stories and traditions.