Seen a few Zeds with them but I can't see the point. The standard seats you get in a GT spec car are great imo, plus they're heated & leather.
If you're still interested in swapping the seats though the best places to look on here are probably in the 350Z "interior" section; http://www.350z-uk.com/forum/48-interior/page__prune_day__100__sort_by__Z-A__sort_key__last_post__topicfilter__all .....and the "members build threads" here; http://www.350z-uk.com/forum/156-member-build-projects/
Lot's of good traders on here to help you spend cash on the Zed if you're modifying it so best look through the Traders section here; http://www.350z-uk.com/forum/93-professional-traders/
Best mods for the 350Z performance wise are; HFC's (High Flow Cats) or De-Cats, full exhaust system (personal preference), air filter/induction kit (again personal preference as not much bhp difference imo), plenum spacer and the UpRev (remap). There are obviously more things you can do but minimal gains unless you're going down the FI route (turbo charged or Supercharged). Hope that helps.