It depends on what parts you're looking for. You'll have to check several places. The following have what you could call typical replacement items
https://www.specialtyz.com/
https://conceptzperformance.com/
https://www.z1motorsports.com/
There's a chance you may have better luck for larger items (and maybe save some money) by looking for used parts on forums or your friendly neighborhood Craigslist. There's a classified Z section here at NICOclub.com where you can post a wanted ad, as well as at
http://twinturbo.net/nissan/300zx/forum ... forum.html. The twinturbo.net is more Z32-concentrated while NICO covers all of Nissan/Infinity. You have the best chance of success if you cast a wide net.
Before you buy, it would be wise for you to do a lot of research and try to get a glimpse of what you're jumping into with the car. It's a great car indeed, but she'll definitely demand your attention. At a minimun, browse for "what to look for before you buy" write-ups and videos so you're aware of red flags. It is most definiely worth the time and effort to wait until you find one that doesn't immediately drown you in maintenance/repairs.
...ditto what Nolimit says above