Hi,
Grahams have a website
http://www.grahamsmotorcycles.com/ which has contact details and lists of spares.
However I wouldn't bother with original parts for a M/C. You should be able to use any old one that is close to the same size and fits the bars. So check out local breakers or ebay.
Personally I replaced my M/C and brake lever assembly with one from a Honda CB500F as the M/C is 1/2", it was a single disc brake, and the huge bonus is that the lever is adjustable for reach.