I've just bought a GPZ500 radiator for £50. I'll need to get the filler and suitable mountings welded on though. I have a TIG welder but might get my mate to do it because I'll no doubt f*@k it up.
http://www.ebay.co.uk/itm/Aluminium-Racing-Radiator-For-Kawasaki-GPZ500S-GPZ500-Ninja-500R-1994-2004-Pro/182632299071?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649