Need help with a diode.

[richards72chevy]

I'm having a problem of my 66 turning over with key in the off position.I figured it may be my switch but further reading on here suggest I need a diode.I have covereted over to an internal regulated alternator a few years ago and never had this problem.I just put in a electric cooling fan and now have this problem.I definitely am going to put a diode on the alternator.Do I need a diode anywhere else? From what I have read on here the diode is a simple cheap fix but all the diodes I'm seeing on the auto zone website are about 50 dollars plus and they all look a little different.Does any one have a picture or a part number to the diode I need? Thanks.