After completing Flavor Country, 'talk' to Chad again for the follow-up mission, Grill Streak.
A fantastic place for this one is the stretch of road north of McCullough's Garage in Faith's region.
One of the Kimberlites is fine for this, and the bison are plentiful both in the nearby fields, and often on the road itself.
Use your Ultimate Hunter homeopathic to mark them, then get driving. Note that bison will take several hits to kill, but will ragdoll and act dead after each collision. Don't be fooled- every time you knock one down, be immediately backing up and lining up your next charge.
Down and loot three bison, then return to Chad for mission completion. You'll win some cash and a new vehicle unlock for your troubles.