Pinballlife was selling them for about $85 around 9 months ago but has been sold out for a while. Hopefully they will make them a regular stock item soon.

Robert, if you don't have a free game level set in your game as an extra ball, then you can probably get the knocker to go off at whatever score you set free game to be. I have it that way in one of my three games so it knocks at the extra ball/free game score, and the other two games are set to extra balls so they don't knock when the extra ball is awarded.

You can extend the wire and run it to the head. You can also mount the coil on a small block of wood and have it hitting the side cabinet and it will be louder. I think MrWizzo had a thread that showed doing that. I will link it in a minute after I find it.
Here it is:

http://pinside.com/pinball/forum/topic/pbl-stern-knocker-placement