Your local dealer can order a lot of parts if you have to part numbers. This link is a great resource. In Canada they can order much more that the US dealers. http://www.carpartsmanual.com/datsun/Z-1969-1978
I have used this one for parts, they have a limited selection, but with free shipping on orders over $100, it is one of the better ones.
https://www.autopartsway.ca/?a_aid=e5f0619e
wheee! is correct some stuff you will only be able to get down south, so be ready to pay shipping/exchange and duty.
Lordco also can source quite a few aftermarket parts, but I find their prices a little higher than online.
Most of your used or hard to find parts are going to come from members here.