I had the same problem on my 600, couldn't find anything decent for ages. Ended up buying a race back end cheap from ebay and cutting it to bits to make my own. If your're handy with fibreglass and filler it'll be no problem...messy tho!!
This was the original one from ebay
And this is the nearly finished seat, needs proffesional spray job!!
There are loads of race seats cheap on ebay, don't matter if they are damaged if you're gona cut them up either.
Billy