If you have not got a new cistern by now, then have a look in the bottom of the cistern to see if any (broken) bits have fallen there. They might show what piece is missing. In my experience, sometimes you might be able to craft a replacement, but buying parts is not possible. A new cistern has usually been the result. Just be sure the replacement you fit is level, or the mechanism might not work properly (they run on water levels). Good luck