There are 20 buffalo that don't respawn so once you kill them they are dead.. mostly found in the great plains. I personally tried to complete this achievement before completeting the mission "For Purely Scientific Purposes" and i managed to kill 18 buffalo but couldn't find anymore even using bait .. so i gave up and continued with the story .. when i got to the mission "For Purely Scientific Purposes" at the beggining you are riding with mc and and nasada . You come across what i guess is supposed to be a heard of buffalo they comment on it.. but in my case there was only two running i popped them both that was it.
Hope this helps. in short you need to complete that mission before you have access to all the buffalo