Spring in the drivers door, to hold it in position, is missing. Can I use any old spring, or do I need a door spring from a Z? Where would you get a door spring? Didn't see one in MSA, VB or Tweeks.

I would use a spring from a Z. It will have the proper size and tension for the weight of the door. You should be able to find one at a junkyard or maybe someone here that is stripping a car out for parts. Been a lot of them losing their life lately due to too much rust seems like.

Is there an easy way to get it out, or do you have to take the whole door off?

Never had to do it, but I would say you could probably do it without removing the whole door. You could try to block the rear of the door to hold it up(maybe with a jackstand) and just remove the hinge itself. There should be someone here with a complete door hinge from a parts car, you could try placing a wanted ad in the classifieds here.