The rear upper wishbones are sold as a single unit, I've not seen any aftermarket ones anywhere for the rear uppers. DO you know what has actually gone? The upper wishbone has two rubber bushes at the inner end, and a ball joint at the outer end. It may be possible to get a rubber bush from something else to fit, but no idea if the ball joint can be replaced seperately. Only sources I can think of off the top of my head are Lexus or breakers yards. I would assume if there are after market ones available, your local motor factor shop will be able to tell. Blueprint make front uppers, not sure about rear uppers though.