Hello Brian,
You can get them new from a Nissan Dealer as I did which will cost you around £27 odd and you will get a nice new cap along with the plastic strap or if you want a cheaper option then try a PM (private message) to zmanalex to see if he has any good second hand ones for you.
Link: http://www.350z-uk.com/user/9648-zmanalex/
If he doesn't have one then try Clark Motorsport as they where breaking two 350 Zeds as was/is R35LEE, Shades Rs
http://www.350z-uk.com/topic/87071-breaking350zs-all-parts-available-cms/
http://www.350z-uk.com/topic/63996-2004-350z-breaking-all-parts/
http://www.350z-uk.com/topic/87886-53-nissan-350z-gt-breaking-spares-with-20-khans/
Hope that helps.