You should also find the hand crank grinders at about any farm store like Orscheln or Tractor Supply and possibly even a Wal-Mart, but you can't count on Wal-Mart for much. An electric grinder may cost a bit more but you can probably find those too. You can go to any locker and ask for pork, beef or whatever fat to add to your burger, or the meat counter in any large store that cuts their own meat and they can tell you how much is reccommended. You can also add sausage because its mostly fat, but your deer will taste like sausage, obviously. But basicly you decide how much lean meat you want and you can find that out from any hamburger you've purchased at a store, by reading the package. Most common is 80/20 If this is what you like then add 2lbs. of fat to 8lbs. of deer and grind and mix well. After the first batch fry up some deerburger and make sure it cooks and tastes right and adjust if necessary.
Another thing you can do is just grind it lean and add lard/crisco to your pan at cooking time like grandma used to do. It does excactly the same thing, it adds the desired amount of fat for flavor and easy cooking without burning. You could simply figure out how many teaspoons of your favorite flavor of cooking lard to add.
Many does have a nice 1/2 inch thick layer of fat covering their hind quarters and over their back straps and you can save it for grinding too.