It can vary. Some are secured using original mountings, some need holes drilling and some are just glued or adhesive tape fixed in place.
Plenty of options from the traders on here: http://www.350z-uk.com/forum/93-professional-traders/
Or if you google it (for example): http://www.epracing.co.uk/index.php/body-kits/aero-kits/nissan/350z/711/spoilers.html
Some related threads:
http://www.350z-uk.com/topic/61020-with-or-without-spoiler/
http://www.350z-uk.com/topic/63754-to-nismo-spoiler-or-not-to-nismo-spoiler/
If you have a look at some of the Zeds on here in the build threads section maybe it'll give you some inspiration. The Nismo V2 spoiler is pretty popular and that'll give you the gap you like or maybe you might like a BGW as some members do.
That's "Big Gay Wing" btw (not hating, that's just what they're called).