Some of the better performance Z sites include
http://www.splparts.com . They have a nice array of Z32 performance stuff.. its pricey, but all Z32 stuff is! Id start with SPL since theyre a NICO sponsor and they have all top quality stuff!
http://www.z1performance.com has stuff too, theyre probably the best Z32 performance site out there, they do a lot of stuff.
IM sure NOPI has some things, but may not be hardcore enough... same thing with
http://www.sportcompactonly.com