A few sites to look at which currently have, or sometimes have RS's for sale:

http://www.two-strokes.com/forums/index.php
http://www.fatbaq.com/mainpage.phtml?topic=news
http://www.rmdmotors.com/
http://www.racing-motorcycles.co.uk/forsale.php
http://www.siegertyamaha.com/motorrad.html
http://www.rscycles.com/Bikes/customersbikes/customer.htm
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30