you won't find too much for a z31na, but for starters you can try the things on <a target=x href=http://z31.com/modifications.shtml>this</a> page.

k&n filters can be had from <a href=http://strictlyZ.com>StrictlyZ</a>, throttle bodies can usually be found at junkyards for a reasonable price, and you might get quotes for exhaust work at some local shops. If you really want to delve into more extensive things like cams and such, then I believe jwt, nismo, isky and various others have that sort of thing. there are probably links on the modifications page (first link in this reply). Poke around on StrictlyZ and see if they have the suspension items too.