I would just buy the ammo from wherever you can get the best deal. I like Midway, but there are other sites that sell ammo as well.
As far as sighting in, assume a 180 gr bullet at 3,100 fps: sight your rifle 3.4" high at 100 yds and it will be 4" high at 200 yds, and have a MPB of 350-ish yds. That does not mean you can't hit anything beyond that yardage, it just means that out to 350 yds you can just hold dead on and shoot and expect to hit the animal. Beyond that range you will have to hold over the desired point of impact (not holding over the animal). Pretty simple, try it.