Hello and welcome to the forum.
OEM springs are Sachs which are readily available in the UK ( don't know about in US).
When you are changing the springs consider changing shock absorbers and as many of the suspension bushes as you fancy. As you have found many of the rubber parts deteriorate and the bushes are no exception. Also take alook at the belts for water pump and air con, if they have not already been changed they could probably do with it.
Enjoy your Z, you have some great back roads in AZ.