As Colin said either refurb your current one, ....buy a good used condition one from zmanalex or you could replace with this aftermarket replacement part that I know several other members have done.
Link: http://www.ebay.co.uk/itm/Ultra-Racing-Rear-Lower-Brace-Nissan-350Z-3-5-2003-RL4-1259-/271339756751?pt=LH_DefaultDomain_3&hash=item3f2d1c1ccf
Personally I'd firstly try using a wire brush on your current one and then painting it with black Hammerite paint.