Hi Harry,
All the recommendations above are great for OEM but personally I'd take the advantage to replace it with a better aftermarket part if you're changing it anyway ~ plus the aftermarket part is cheaper.
Link: http://www.ebay.co.uk/itm/Ultra-Racing-Rear-Lower-Brace-Nissan-350Z-3-5-2003-RL4-1259-/271339756751?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3f2d1c1ccf
Have a good look on eBay though as the same brace above was up for £129 just a few weeks ago from another seller but couldn't find it when I just looked. Good luck & hope that helps.