I presume you are talking about the 501s and if so they show up from time to time on ebay. The are rare and often snapped because of their nikasil engine for a replacement with earlier street morini bikes.
if you are talking about the first series (MK1-MK2 and MK3) then it is almost impossible to find one and the ones i know are in the hands of collectors or Morini big time afecionados. BTW I sold mine few months ago after i posted on the for sale section.
Cheers and good luck