Bison are easily located in the Henbane River
region of the map, near the southeast. Locate road signs to have their locations marked to your map.
Once you find a bison, you should be able to easily kill it with a melee weapon in only 10-15 hits. However, if you'd like to make it even easier open your weapon wheel, switch to utilities, then craft and consume "The Furious" potion. This should allow you to kill the bison in only 1 hit.