does anyone now of an aftermarket wastegate that makes the fluttering noise when lifting off the accelorator ive heard these on skylines and other jap cars

 

That's usually not the wastegate: it's usually the gradual death rattle of the turbo stalling. It sounds really cool, but it doesn't do a lot for turbo life. Usually down to no dumpvalve / dumpvalve closing too early / dumpvalve not flowing enough.