I was told when I worked at Philmont Scout Ranch [1] that they had a pure herd.

Other than that, I know Steven Rinella listed a few pure herds in his (excellent) book [2] on the American Buffalo, but I'd have to dig it out to find them for you.

[1] https://www.youtube.com/watch?v=jzyMrBUys90

[2] https://www.amazon.com/American-Buffalo-Search-Lost-Icon/dp/...

ETA: https://www.nps.gov/articles/000/wind-cave-bison-genetics.ht...

"Wind Cave and Yellowstone National Parks are the only two federal herds to have population sizes large enough for sufficient testing. Both herds show no evidence of cattle introgression."

So there are at least two

A plug for Steve Rinella's excellent book, "American Buffalo: In Search of a Lost Icon".[0]

He recounts his grueling trip in the Copper River Basin in Alaska on a hunt for a bison, and every other chapter continues a parallel telling of the history of the species in North America, particularly the archaeological evidence of prehistoric hunting of the animal, from large kill sites to fascinating sites like the Mesa site in northern Alaska[1] linking arctic cultures to Paleoindians in midcontinent.

It's a really enjoyable book, and he narrates the audio book.

[0] https://www.amazon.com/American-Buffalo-Search-Lost-Icon/dp/...

[1] https://www.blm.gov/programs/cultural-heritage-and-paleontol...