That is definitely the worst condition W-brace I've seen on a 350 Willow.
Usually the cheap option that most people can do is remove it, clean the rust off and "Hammerite" paint over it but yours definitely needs binning asap before it falls off in the road, ...could be especially bad if it comes off and hits another car.
You could try zmanalex on here for a pre-owned W-brace which is the cheapest option.
Link: http://www.350z-uk.com/user/9648-zmanalex/
Your next option and imo the best option is to get an aftermarket W-brace as it will be cheaper than getting a Nissan OEM replacement. This is the one I usually recommend to people on here and everyone I know has been more than happy with it.
Link: http://www.ebay.co.uk/itm/Ultra-Racing-Rear-Lower-Brace-Nissan-350Z-3-5-2003-RL4-1259-/271339756751?hash=item3f2d1c1ccf
If you don't like or want it though then I believe the Cougar Store or Clark Motorsport on here can both get you the OEM W-brace. Other traders may be able to as well but they are just the two I know who definitely should be able to although if memory serves it's around the £200 mark for the OEM one.
Good luck.