You can only find the Golf Club during 1 mission, which is 'Anathema' and it is only carried by 1 person, Giuseppe Guillani. You must kill Giuseppe Guillani to take the Golf Club from him, then when it is in your possession, you must proceed to kill 18 people with it. It is strongly recommended to attempt this on an easy difficulty, as you will be trying to kill armed guards with it, who will not hesitate to shoot you.
The 18 kills is cumulative, so you can get this by getting 10 kills, then playing the mission again and getting 8 kills, etc. The kills will only count if you finish the mission though. If you quit out or restart the mission, they will not count.