Best place to try would be Robinsons Foundry
http://www.robinsonsfoundry.co.uk
I've had a few suzuki parts from there before.
They do list some gsxr part, including a top cap -
http://www.robinsonsfoundry.co.uk/sh...350/index.html
probably best to send them a pic and see what they say.
Cheers
Dave