Another term for "origin story" is "creation myth" (and there may be other terms as well), so try different searches!
You can also just do a broad search for your nation and explore the official websites.
Check YouTube for the official channels of nations or tribes.
If you're unsure about whether the origin story is authentic, compare what you find from a few different sources. If they're all similar, chances are, they are legit!
The general resources on the Library Resources and Online Resources pages of this guide may also be useful. If you have trouble searching inside the websites, just look for links in your Google results that come from them.