Exterior replacement panels and patch panels:
http://www.victoriabritish.com/
http://www.zcarparts.com/
If you need replacement rails/pans for, um, more "aggressive" repairs...
http://www.datsunzparts.com/
And Courtesy Nissan can be a good source, too:
http://www.courtesynissan.com/

Of course, all of the above are call-and-order, so you definitely *pay* for the parts. But, they're new and usually very nice steel parts (I've had good luck, at any rate). Cruising the salvage yards and swap meets can save you a good bit of cash, but of course you may not find a part up to your standards. Not sure if Arizona Z car still has any fiberglass replacment pieces available, http://www.arizonazcar.com/. Anyone know for sure?
Good 'ole Z body work. Rust disintegrates our cars, but definitely keeps the Z community together! Who here hasn't spent at least a little time battling the tin worm?

Dave