The National Park Service site found by Masato is nicely set up for mp3 copying of the material that they have prepared. I hope that more will be added in the future. It might be worth writing them from time to time to encourage additions- they say they have 35000 items in this collection.

Out of curiosity I tried to locate the recordings by starting with www.nps.gov and attempting to find it through their index, but no luck. Have you tried to find the path to these pages that way?

They have as well a fine old Dalhart there (Get out of my way) that I was very glad to find.