If you want to see Bison in Yellowstone NP then Hayden Valley is the area you want to drive through. There always seem to be at least a few herds in the valley. Don’t forget to bring your binoculars since some of the Bison may not be close to the road and other animals visit the valley too.
BTW, it always a good idea to watch your step in areas that Bison frequent. Buffalo chips are everywhere.