don't try it yourself unless you know how to set it up. it isn't just a matter of changing out the ring and pinion...there are critical adjustments that most be set. then once that is done, depending on year and make, you may have to have the enine management computer reset or "flashed" for the change in gear ratio......
if you can't do it yourself. it ain't cheap to do.....
as far as the needed parts, most any dearler or good parts house can get them.....