There are several parks where the bison (aka buffalo) roam. There are herds in the following national parks:

Theodore Roosevelt NP
Badlands National Park - reintroduced
Capitol Reef National Park - reintroduced
Grand Teton National Park
Wind Cave National Park - reintroduced
Wrangell-St. Elias National Park - introduced

Bison used to roam across the entire prairie and were extirpated from the following areas that are now part of the national park system:

Rocky Mountain National Park
Guadalupe Mountains National Park
Great Smoky Mountains National Park
Great Sand Dunes National Park