I looked into this sometime back and even got plans from a company in New Hampshire. Not sure I can find it now and I may have given them to someone else.
I was not impressed with the plans and unlike professional shocking equipment the voltage could not be regulated and it came off of batteries. It was implicitately stated that that the apparatus could actually kill the fish it shocked.So not wanting that to happen I abandoned building one myself.
Even modern equipment requires an adjustment for the conductivity of the water. There is a company that makes electroshockng equipment but you are talking several thousand for the basic equipment if I remember right. I believe the main expense is the converter box.
Here's the supplier:
http://www.smith-root.com/products/