I just tried to start my '67 mustang and the starter solenoid started clicking.  I had an extra one so I replaced it and then the car started.  Can someone tell me causes the solenoid to go out and who sells the best quality solenoid?  

The solenoid is a big relay.  It has a copper piece shaped like a large washer that gets lifted by an eletric magnet to make contact between the battery (one large lug on the solenoid) and the other large lug and the I terminal.  The copper washer and the large lug contacts pit after years of use and it loses the ability to make a good electrical connection.  Naturally, it is riveted it together and you can't repair it.  I don't know who makes the best one.  I just buy a name brand part and go with it.  These days some foreign entity is making almost everything, anyway.  My American made, three year old, genuine reproduction voltage regulator just croaked.  I replaced it with a $10 solid state Auto Zone part.  They were open on Sunday and I wanted to drive my Mustang.  

I don't know of a name brand solenoid that would be any better than any other.  I bought mine from AutoZone about 15 years ago and she is still ticking.