Ebay UK list them as available from USA:
https://www.ebay.co.uk/itm/Yamaha-XS650-70-83-Kibblewhite-Valve-Spring-Set-None-80-80344/151788102373?hash=item235746a2e5:g:~LsAAOSwrklVAFE7&_sacat=0&_nkw=xs650+valve+springs&_from=R40&rt=nc&_trksid=m570.l1313
After Market.
https://www.ebay.co.uk/itm/Yamaha-XS650-70-83-Kibblewhite-Valve-Spring-Set-None-80-80344/151788102373?hash=item235746a2e5:g:~LsAAOSwrklVAFE7&_sacat=0&_nkw=xs650+valve+springs&_from=R40&rt=nc&_trksid=m570.l1313
After Market.